fix(agent): treat only an omitted case as a directory spawn, and document the directory launch

`codeman agent spawn ""` (for example `spawn "$CASE"` with CASE unset)
used to launch a worker in the caller's cwd. Both case checks now test
`caseName !== undefined`, so an empty name reaches quick-start and is
refused by its schema as before.

Drop the `{ id }` fallback on the create answer: `POST /api/sessions`
answers `{ session: { id } }`, and the test fakes now use that shape.

Tests: an empty case name (unit and through the real argument parser
and HTTP transport), the claude `--ready` order create, /interactive,
wait-output on the created id, and a non-JSON failed start (id alone on
stdout, reason on stderr, exit 1).

Docs: the spawn mapping and directory-launch JSON shape in
docs/api-reference.md, the README and wiki `codeman agent` sections, the
cli-agent @fileoverview and the skill reference tables; reflow the new
SKILL.md paragraph (plugin mirror synced); the changeset notes the
local exit-4 refusal of an unknown mode for named cases.

docs/api-reference.md

@@ -591,7 +591,7 @@ request returns. The capture runs synchronous tmux calls on the server; the
 
 ## The `codeman agent` CLI (client over these endpoints)
 
-`codeman agent ls|spawn|send|wait|read|interrupt|rm|post|inbox|ack` (`src/cli-agent.ts`) is the command-line client for the endpoints above, for agents the skill does not reach (a pi or shell session, a machine without `jq`). It adds no route: `spawn` is `POST /api/v1/quick-start` (+ `wait-output` on the mode's `capabilities.composerReadyMark` from the CLI registry, where it declares one), `send` is `POST …/input` with `clientId`+`seq` (and `wait`/`waitTimeout` for `--wait` / `--until <signals>`; `delivered:false` without `duplicate` and `wait.ended` both exit 3 — the CLI never reports a dead worker as done), `wait` is `GET …/wait` (`--until`) or `GET …/wait-output` (`--match`, `from=buffer` by default), `read` is `GET …/last-response` or `GET …/terminal?tail=`, `interrupt` is `POST …/input` with a bare `\u001b`, `rm` is `DELETE …/sessions/:id`, and the inbox verbs map one to one onto the [agent inbox](#agent-inbox) routes (`post` → `POST …/inbox`, `inbox [id] [--wait]` → `GET …/inbox[?wait=]`, own inbox by default, `ack <ids…>` → `POST …/inbox/ack`); `ls` adds an `INBOX` column from `GET /api/v1/agent-inbox/summary` and drops it when that route does not answer. A fire-and-forget `send` to a sleeping wake-on-LAN host reads the route's `buffered` (own line, exit 0) and `dropped` (exit 1: the chunk is gone). An id may be the 8-character form `ls` prints, resolved through `GET /api/v1/sessions`; anything shorter refuses before any request, the same floor as `PARENT_SESSION_ID_MIN_PREFIX`. Every call carries `X-Codeman-Parent-Session`; only `spawn`'s quick-start carries `X-Codeman-Agent-Origin: codeman-agent-cli` (the agent-scratch label must never reach a request that cannot create the case directory). Basic auth comes from `CODEMAN_PASSWORD` or the data dir's `.env`. Server-side error codes are shown verbatim (`INVALID_INPUT: until=stop …` on a hook-less mode is not hidden); exit codes are `0` ok, `1` error, `2` timeout, `3` the session exited, `4` refused by a client-side guard (for `inbox --wait`, `3` only when `GET /api/v1/sessions` no longer lists the session; a wait the server ended otherwise, on a restart or a detach, is `2`). `post` takes text that starts with `-` as it is (a markdown file often does); text that is exactly an option (`--json`, `-h`) goes after `--`. See the README section "`codeman agent`" for the guards and `test/cli-agent.test.ts` for the pinned behaviour.
+`codeman agent ls|spawn|send|wait|read|interrupt|rm|post|inbox|ack` (`src/cli-agent.ts`) is the command-line client for the endpoints above, for agents the skill does not reach (a pi or shell session, a machine without `jq`). It adds no route: `spawn <case>` is `POST /api/v1/quick-start`, and `spawn` with no case (the caller's cwd) or with `--working-dir <dir>` (an existing local directory; combining it with a case refuses) is `POST /api/v1/sessions` with `workingDir`, `mode`, `name` and `parentSessionId` followed by `POST /api/v1/sessions/:id/interactive` or `/shell` (the mode's `capabilities.startMode`), never retried; either form then runs `wait-output` on the mode's `capabilities.composerReadyMark` from the CLI registry where it declares one. `send` is `POST …/input` with `clientId`+`seq` (and `wait`/`waitTimeout` for `--wait` / `--until <signals>`; `delivered:false` without `duplicate` and `wait.ended` both exit 3 — the CLI never reports a dead worker as done), `wait` is `GET …/wait` (`--until`) or `GET …/wait-output` (`--match`, `from=buffer` by default), `read` is `GET …/last-response` or `GET …/terminal?tail=`, `interrupt` is `POST …/input` with a bare `\u001b`, `rm` is `DELETE …/sessions/:id`, and the inbox verbs map one to one onto the [agent inbox](#agent-inbox) routes (`post` → `POST …/inbox`, `inbox [id] [--wait]` → `GET …/inbox[?wait=]`, own inbox by default, `ack <ids…>` → `POST …/inbox/ack`); `ls` adds an `INBOX` column from `GET /api/v1/agent-inbox/summary` and drops it when that route does not answer. A fire-and-forget `send` to a sleeping wake-on-LAN host reads the route's `buffered` (own line, exit 0) and `dropped` (exit 1: the chunk is gone). An id may be the 8-character form `ls` prints, resolved through `GET /api/v1/sessions`; anything shorter refuses before any request, the same floor as `PARENT_SESSION_ID_MIN_PREFIX`. Every call carries `X-Codeman-Parent-Session`; only `spawn`'s quick-start carries `X-Codeman-Agent-Origin: codeman-agent-cli` (the agent-scratch label must never reach a request that cannot create the case directory). Basic auth comes from `CODEMAN_PASSWORD` or the data dir's `.env`. `--json` prints the envelope's `data`, except a directory `spawn`, which prints `{ sessionId, workingDir, created: true, ready, readinessError }`, and on a failed or ambiguous start `{ sessionId, workingDir, created: true, ready: false, startupError }` (exit 1, the created id kept so the caller can inspect it). An unknown `--mode` refuses (exit 4) before any request, for a named case too. Server-side error codes are shown verbatim (`INVALID_INPUT: until=stop …` on a hook-less mode is not hidden); exit codes are `0` ok, `1` error, `2` timeout, `3` the session exited, `4` refused by a client-side guard (for `inbox --wait`, `3` only when `GET /api/v1/sessions` no longer lists the session; a wait the server ended otherwise, on a restart or a detach, is `2`). `post` takes text that starts with `-` as it is (a markdown file often does); text that is exactly an option (`--json`, `-h`) goes after `--`. See the README section "`codeman agent`" for the guards and `test/cli-agent.test.ts` for the pinned behaviour.
 
 `codeman mcp` (`src/mcp-server.ts`) is a Model Context Protocol server on stdio over the same endpoints, for MCP clients: `list_sessions` (`GET /api/v1/sessions`), `start_session` (`POST /api/v1/quick-start` with `X-Codeman-Agent-Origin: codeman-mcp`, then `wait-output` on the mode's ready mark), `send_prompt` (`POST …/input` with `wait`/`waitTimeout`), `wait_for_session` (`GET …/wait` or `…/wait-output`), `read_output` (`GET …/last-response` or `…/terminal?tail=`), `interrupt_session` and `delete_session`. It adds no route. The URL comes from `CODEMAN_API_URL` or `--url`, never a guess; prompting, interrupting and deleting are limited to sessions the process started (or the calling session's own children) unless it runs with `--allow-any-session`. See the README section "MCP server" and `test/mcp-server.test.ts`.