feat(agent-cases): tag agent-spawned case dirs and sweep their leftovers

A long orchestration creates one case directory per worker and deleting the
sessions never removed them, so ~/codeman-cases accumulated scratch folders
that were indistinguishable from real projects. They are now labelled and
have a cleanup path.

- src/agent-case-marker.ts: a case dir quick-start CREATES for an agent-driven
  spawn gets a .codeman-agent-case.json marker (when, by whom, parent session,
  mode). Only the create branch writes it, so a linked case, a cloned repo or
  any pre-existing path is never labelled; reading is total, so a malformed
  marker means "not agent-created" rather than a half-trusted entry.
- The signal is the new X-Codeman-Agent-Origin header the skill preamble sets
  on its shared curl (preamble bumped to 1.22.0), or an agentOrigin body
  field, falling back to a resolved parentSessionId so a worker spawned by a
  stale skill copy is still labelled.
- GET /api/cases publishes it as agentCreated; GET /api/cases/agent-created is
  a read-only cleanup listing adding inUse and modifiedAt; Add Case -> Manage
  badges each case and offers a review-then-delete sweep that names every
  directory in its confirm and skips any case a live session is working in.
  Removal stays on the existing DELETE /api/cases/:name.
- Agent preamble caches are collected too: ~/.cache/codeman-agent-<id>.sh was
  written per claude session and never removed (236 leftovers measured on a
  working machine). Now deleted with the session and swept at boot, guarded by
  a live-session keep set plus a 7-day age floor.

Verified end to end on an isolated instance: marker written for header, body
and lineage-only spawns, absent with no agent signal and for a pre-existing
directory; inUse flipping on session end; badge, sticky bar, confirm and sweep
driven in a browser; preamble seeded on create, removed on delete, boot sweep
taking only the aged orphans.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

skills/codeman/reference/verbs.md

@@ -731,6 +731,21 @@ Deleting a session ends the agent and its pane. It does **not** remove:
   it, and ask before running `git worktree remove`, which discards uncommitted work
   inside it.
 
+Those case directories are **labelled** rather than left anonymous. A directory
+`quick-start` creates for a spawn carrying the preamble's `X-Codeman-Agent-Origin`
+header gets a `.codeman-agent-case.json` marker, which is what puts it in the web UI's
+agent-case cleanup list (Add Case → Manage) and in:
+
+```bash
+"${CURL[@]}" "$API/api/v1/cases/agent-created" | jq -r '.data.cases[] | "\(.name)\t\(.createdAt)\tinUse=\(.inUse)"'
+```
+
+Read-only, scoped to the user's own case space, and `inUse` is true while a live
+session is still working in that directory. Report that list when you finish a run
+with workers, so the user knows exactly what to sweep; the deletion is still theirs to
+ask for by name. Only a directory Codeman **created** is ever labelled, so a linked
+case, a cloned repo or a worktree never appears there.
+
 Confirm cleanup with `GET /api/v1/sessions`, never with `/api/v1/sessions/unified`
 (that one folds in transcript history from the whole machine and will keep showing
 your worker forever).