feat(cases): clone a repository into a custom folder

Clone Repo could only clone under ~/codeman-cases. POST /api/cases/clone now takes an optional path (parent folder plus case name) and the Clone tab gets a Clone into a custom folder option with a Browse button, mirroring Create in a custom folder. The path is judged by prepareNewCasePath, the clone is registered as a linked case, the option is admin-only in multi-user mode, and an existing target folder is refused.

docs/architecture-invariants.md

@@ -468,7 +468,7 @@ Tests: `test/file-preview-markdown.test.ts` (jsdom-in-vm, pins every rule above)
 
 ### Clone a repository as a case
 
-**Clone Repo tab** (issue #236, proposed by @DodgyBadger): `POST /api/cases/clone` clones a public repository into the caller's case space and registers it as a normal local case; `POST /api/cases/clone-preflight` answers "can this be cloned anonymously, and what refs does it have?" while the user is still typing. Core in `src/git-clone.ts`, split into a PURE half (URL parse, argv/env, `ls-remote` parse, stderr classification) and a thin IO half (`probeGitRemote`, `cloneRepository`).
+**Clone Repo tab** (issue #236, proposed by @DodgyBadger): `POST /api/cases/clone` clones a public repository into the caller's case space and registers it as a normal local case; `POST /api/cases/clone-preflight` answers "can this be cloned anonymously, and what refs does it have?" while the user is still typing. Core in `src/git-clone.ts`, split into a PURE half (URL parse, argv/env, `ls-remote` parse, stderr classification) and a thin IO half (`probeGitRemote`, `cloneRepository`). An optional `path` (the **Clone into a custom folder** checkbox, parent folder plus case name) clones into that exact folder instead and registers it as a LINKED case, the same way `POST /api/cases` with a `path` does: it is judged by `prepareNewCasePath()` (`web/case-path.ts`: blocked trees, symlinks, parent must exist), is admin-only in multi-user mode (it writes an absolute path into the shared linked-cases registry), and refuses a folder that already exists, empty or not, because `cloneRepository` needs a fresh destination. Deleting such a case unlinks it and never removes the files. If the registry write fails after git finished, the folder is kept and the error says where it is.
 
 - ⚠️ **The URL is a code-execution surface, which is why it is parsed rather than forwarded.** `ext::sh -c <cmd>` makes git run an arbitrary command as its transport, and ANY `<name>::<payload>` dispatches to a `git-remote-<name>` helper, so every `::` form is refused outright. A repository starting with `-` is read by git as a flag; that is rejected AND every spawn puts `--` before the operands, because either defence alone is one edit away from being a hole. Spawns are argv arrays, never a shell (unlike `remote-hosts.ts`, which does build a shell line and must `shellescape`). The Zod schema deliberately only length-bounds `repository` — a weaker regex duplicate of `parseGitRepositoryUrl` would be the copy that drifts.
 - ⚠️ **Non-interactive or it hangs the request.** The clone is synchronous by design (no job store, no polling, no cancellation surface), so an invisible credential prompt would pin an open HTTP request until the timeout. `gitNonInteractiveEnv()` closes all four prompt paths at once: `GIT_TERMINAL_PROMPT=0`, empty `GIT_ASKPASS`/`SSH_ASKPASS` + `SSH_ASKPASS_REQUIRE=never` + empty `DISPLAY`, `GCM_INTERACTIVE=never`, and `ssh -oBatchMode=yes`. `HOME`/`PATH` are inherited on purpose — a user whose own agent or credential helper already works keeps working (so a private repo may well clone; Codeman just never collects or stores credentials, and refuses a `user:password@` URL). ⚠️ In multi-user mode a NON-ADMIN's clone and preflight run with `git -c credential.helper=` (`cloneWithoutCredentialHelpers`), because the helpers belong to the one server account every user shares; admins and single-user mode keep them. The Docker server image relies on exactly this: when built with the opt-in `CODEMAN_INSTALL_GH`/`CODEMAN_INSTALL_AZ` args it configures `gh` (github.com) and an `az`-backed helper (`docker/git-credential-azure-cli`, Azure DevOps) in its SYSTEM gitconfig, both silent until the user signs the CLI in, so a private clone there still fails fast rather than prompting.

docs/wiki/Core-Concepts.md

@@ -25,7 +25,7 @@ Three ways to get one, all under **New or link a case…** at the bottom of the
 | How               | Result                                                                                                                                                                                                              |
 | ----------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | **Create New**    | A fresh `~/codeman-cases/<name>` with a scaffolded `CLAUDE.md`, or with **Create in a custom folder**, a new folder inside a parent you choose, scaffolded the same way and registered in place like a linked case. |
-| **Clone Repo**    | A repo cloned into `~/codeman-cases/<name>` and registered as a case. Private repos need this machine's own git credentials (see below).                                                                            |
+| **Clone Repo**    | A repo cloned into `~/codeman-cases/<name>` (or into a folder you choose) and registered as a case. Private repos need this machine's own git credentials (see below).                                              |
 | **Link Existing** | An existing folder anywhere on disk, registered in place. Nothing is copied or moved.                                                                                                                               |
 
 Linked cases keep living where they are. Deleting a case in Codeman removes the