fix(cases): serialize every write to linked-cases.json

Five routes changed the linked-cases registry: link, unlink, rename, and a
create or clone into a custom folder. Each read the file, checked it and wrote
it back on its own, so two requests that both read before either wrote lost
an entry. Two clones that claimed one name and finished together both passed
the name check, both answered success, and the later write replaced the
earlier entry. The same race made "a name linked while the clone ran is a
409" in test/routes/case-clone-custom-folder.test.ts fail about half the time.

- New src/web/linked-cases-writer.ts, mutateLinkedCases(), shaped like
  mutateSettingsFile(): mutations run one at a time, each write is a temp
  file renamed over the registry (through a symlink, keeping its mode), and
  a file that is not a JSON object is refused (409 CONFLICT) instead of being
  read as {} and overwritten.
- All five writers go through it, with their name checks inside the
  callback, so exactly one of two requests for one name wins. Rename also
  re-checks that the old name still points at the old folder, and drops its
  manual registry undo, since a failed write no longer changes the file.
- Tests: test/linked-cases-writer.test.ts (writer behaviour, and concurrent
  link/unlink through the routes with slowed writes, which fail on the old
  code). Tests that forced a failed registry write with chmod 444 or a
  directory now inject the failure at the rename, or expect the refusal.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -183,7 +183,7 @@ Codeman is a Claude Code session manager with web interface and autonomous Ralph
 
 **DeepSeek web UI** (`POST`/`GET`/`DELETE /api/deepseek/web`, `deepseek-web-server.ts`): the Run menu's "DeepSeek web UI..." entry supervises ONE background `dsh web` child process, deliberately **NOT a shell session**. ⚠️ Every piece is load-bearing: exactly one server (a second click REUSES it), restarted when the browser authority changes (`--trusted-host`, last asker wins), killed on shutdown via `stopDeepSeekWeb()` (the detached child would otherwise outlive Codeman and hold its port), and failures returned to the caller. ⚠️ Never hardcode the port: search from 3080 across 40, detect free ports by BINDING, then wait for the server to really answer. ⚠️ Both `POST` and `DELETE` must stay behind `canUsernameRunPrivilegedCommands` (booting a profile runs its plugin code; the server is shared). → [architecture-invariants#deepseek-web-ui](docs/architecture-invariants.md#deepseek-web-ui)
 
-**Case rename** (`POST /api/cases/:name/rename`, `src/case-rename.ts`; `docs/wiki/Core-Concepts.md#case`): a case under the cases dir is its folder name and always moves; a linked case renames its alias and moves its real folder only with `renameFolder`; remote and Docker cases are refused. ⚠️ A folder that sessions have open is the hazard: live sessions at or under it make the route answer `409 requiresStop` with their names, and only `stopSessions: true` ends them (via `ctx.cleanupSession`: closed like a tab close, uploads included, nothing is restarted); the free-target pre-check runs before that and refuses a dangling symlink too (`lstat`, as `moveCaseFolder` does), so no session is ended for a move that cannot happen. `withCaseLock` holds the old and new paths for the whole operation (`isCasePathLocked`; the `Session` constructor throws for a working directory inside a held path, so nothing is constructed mid-move). ⚠️ A create awaits the owner's layout lock and a settings read between `new Session` and the server's map, where the rename looks for open sessions, so the constructor also records the session (`noteCaseSessionStarting`) until `registerSessionWithLayout()` or `stop()` settles it (30 s TTL for one that never registers), and a folder move refuses `409` with `renameCode: "BUSY"` while one is recorded at or under the folder (`isCaseSessionStarting`, checked in the same tick that takes the lock); an alias-only rename is not refused. A create whose path was validated before the move but whose `Session` is constructed after it is not covered (the plain check-then-use race of any create path), `moveCaseFolder` refuses an existing target itself (POSIX `rename` would silently replace an EMPTY directory) and retries `EBUSY`/`EPERM`/`EACCES` before answering `423 LOCKED`. If the registries (`linked-cases.json`, `caseOrder`, `lastUsedCase`) fail after the move, `settings.json` and the folder are put back (a failed move-back says the folder is now at the new path), and an error after `stopSessions` carries `stoppedSessions`. Claude Code keys history by path (`claudeProjectDirName`: every non-alphanumeric becomes `-`), so `<claude config>/projects/<old>` is renamed too, or merged entry by entry, never overwriting, into one already at the new key (case delete never removes history, so an earlier case of the new name leaves one); other CLIs' history, Docker bind mounts, `git worktree` links, cron jobs' `workingDir`, Claude history of subfolder sessions and Claude Code's per-project entry in `~/.claude.json` (local-scope `mcpServers`, `allowedTools`, trust; writing another CLI's config is the opt-in MCP sync's job) are NOT migrated. The history scanner prefers the decoded project key over a transcript's `cwd` that no longer exists, so Resume follows a rename. Only an admin's rename touches the server-wide `settings.json` (`caseOrder`, `lastUsedCase`); the owner's default session group is carried across with `TabLayoutService`. `settings.json` is written before `linked-cases.json`, and both are restored if the registry write fails.
+**Case rename** (`POST /api/cases/:name/rename`, `src/case-rename.ts`; `docs/wiki/Core-Concepts.md#case`): a case under the cases dir is its folder name and always moves; a linked case renames its alias and moves its real folder only with `renameFolder`; remote and Docker cases are refused. ⚠️ A folder that sessions have open is the hazard: live sessions at or under it make the route answer `409 requiresStop` with their names, and only `stopSessions: true` ends them (via `ctx.cleanupSession`: closed like a tab close, uploads included, nothing is restarted); the free-target pre-check runs before that and refuses a dangling symlink too (`lstat`, as `moveCaseFolder` does), so no session is ended for a move that cannot happen. `withCaseLock` holds the old and new paths for the whole operation (`isCasePathLocked`; the `Session` constructor throws for a working directory inside a held path, so nothing is constructed mid-move). ⚠️ A create awaits the owner's layout lock and a settings read between `new Session` and the server's map, where the rename looks for open sessions, so the constructor also records the session (`noteCaseSessionStarting`) until `registerSessionWithLayout()` or `stop()` settles it (30 s TTL for one that never registers), and a folder move refuses `409` with `renameCode: "BUSY"` while one is recorded at or under the folder (`isCaseSessionStarting`, checked in the same tick that takes the lock); an alias-only rename is not refused. A create whose path was validated before the move but whose `Session` is constructed after it is not covered (the plain check-then-use race of any create path), `moveCaseFolder` refuses an existing target itself (POSIX `rename` would silently replace an EMPTY directory) and retries `EBUSY`/`EPERM`/`EACCES` before answering `423 LOCKED`. If the registries (`linked-cases.json`, `caseOrder`, `lastUsedCase`) fail after the move, `settings.json` and the folder are put back (a failed move-back says the folder is now at the new path), and an error after `stopSessions` carries `stoppedSessions`. Claude Code keys history by path (`claudeProjectDirName`: every non-alphanumeric becomes `-`), so `<claude config>/projects/<old>` is renamed too, or merged entry by entry, never overwriting, into one already at the new key (case delete never removes history, so an earlier case of the new name leaves one); other CLIs' history, Docker bind mounts, `git worktree` links, cron jobs' `workingDir`, Claude history of subfolder sessions and Claude Code's per-project entry in `~/.claude.json` (local-scope `mcpServers`, `allowedTools`, trust; writing another CLI's config is the opt-in MCP sync's job) are NOT migrated. The history scanner prefers the decoded project key over a transcript's `cwd` that no longer exists, so Resume follows a rename. Only an admin's rename touches the server-wide `settings.json` (`caseOrder`, `lastUsedCase`); the owner's default session group is carried across with `TabLayoutService`. `settings.json` is written before `linked-cases.json`, and both are restored if the registry write fails. ⚠️ Every write to `linked-cases.json` (link, unlink, rename, and a create or clone into a custom folder) goes through `mutateLinkedCases()` (`src/web/linked-cases-writer.ts`: serialized, temp file + rename, and a file that does not parse is refused, never read as `{}`), and a check that must still hold when the entry is written (the name is free) runs INSIDE its callback: two clones that claimed one name used to both pass a re-read made outside it and both answer success. Pinned by `test/linked-cases-writer.test.ts`.
 
 **Custom Model Endpoint Profiles** (opt-in, `customModelEndpointsEnabled`, SYNCED, default OFF; `docs/custom-model-endpoints.md`): points a session at a user-configured OpenAI-compatible endpoint (store `custom-model-hosts.ts`, `~/.codeman/custom-model-hosts.json`, 0600). `authStyle` is `bearer` or `api-key`, never both headers (hangs the server). The per-CLI redirect is registry data, `capabilities.customModelInjection` (`env` / `configContentEnv` / `configDir` / `unsupported`), computed by the pure `custom-model-injection.ts`; ⚠️ `configDir` writes an isolated per-session config, NEVER the user's real `~/.codex`/`~/.pi`/`~/.omp`/grok config; the one exception is a recipe's `linkFromRealHome` (deepseek's `profiles`, `sessions`, `settings.yaml`), which symlinks those named entries back out of the real home so they survive the redirect. ⚠️ Claude applies via `Session.restartCli()` (a relaunch in the existing pane, so the conversation is pinned as `resumeSessionId`), the rest one-shot at launch; retired env keys must also be `setenv -u`'d (`_pendingEnvUnsets`), since tmux env survives `respawn-pane`. ⚠️ Remote/Docker sessions are refused (400). ⚠️ Persist only KEYS (`__customModel`), never values (they carry the API key). ⚠️ Every redirectable var must be in that CLI's `privilegedEnvKeys`, and `ANTHROPIC_*` stays out of claude's `allowedPrefixes`. ⚠️ The Run-menu picker builds entries from `window.__codemanCustomModelClis` (escaped via `escapeScriptJson()`), never a hardcoded CLI id list, and launches through `run()` via a temporary `_runMode` swap, never `setRunMode()`. → [architecture-invariants#custom-model-endpoint-profiles](docs/architecture-invariants.md#custom-model-endpoint-profiles)