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>

src/web/schemas.ts

@@ -1025,6 +1025,16 @@ export const QuickStartSchema = z.object({
   envOverrides: safeEnvOverridesSchema,
   /** Claude CLI effort level (soft default via --settings, switchable in-session via /effort) */
   effort: effortLevelSchema,
+  /**
+   * Who is spawning this worker (`codeman-skill` from the packaged agent skill), or,
+   * equivalently, the `X-Codeman-Agent-Origin` header; the body wins when both are
+   * present. Used ONLY to label a case directory this request CREATES as an agent
+   * scratch workspace, so it can be found and cleaned up later — see
+   * `src/agent-case-marker.ts`. Never a permission signal, and an unrecognised token
+   * is dropped rather than rejected. `POST /api/sessions` has no equivalent field
+   * because it takes an existing `workingDir` and so never creates a directory to label.
+   */
+  agentOrigin: z.string().max(64).optional(),
 });
 
 // ========== Hook Events ==========