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/hooks-config.ts

@@ -1066,9 +1066,80 @@ export async function installAgentSkillInto(skillDir: string): Promise<AgentSkil
  */
 export async function seedAgentSessionPreamble(sessionId: string): Promise<void> {
   const content = await readFile(join(agentSkillSourceDir(), 'preamble.sh'), 'utf-8');
-  const cacheDir = process.env.XDG_CACHE_HOME || join(homedir(), '.cache');
-  await mkdir(cacheDir, { recursive: true });
-  await writeFile(join(cacheDir, `codeman-agent-${sessionId}.sh`), content, { mode: 0o600 });
+  await mkdir(agentPreambleCacheDir(), { recursive: true });
+  await writeFile(agentPreamblePath(sessionId), content, { mode: 0o600 });
+}
+
+/** Where the preamble caches live. One formula, shared by seed / remove / prune. */
+function agentPreambleCacheDir(): string {
+  return process.env.XDG_CACHE_HOME || join(homedir(), '.cache');
+}
+
+/** `codeman-agent-<sessionId>.sh` in that directory. */
+function agentPreamblePath(sessionId: string): string {
+  return join(agentPreambleCacheDir(), `codeman-agent-${sessionId}.sh`);
+}
+
+/** Matches exactly what seedAgentSessionPreamble writes, and nothing else in ~/.cache. */
+const AGENT_PREAMBLE_FILE_PATTERN = /^codeman-agent-(.+)\.sh$/;
+
+/** How long a preamble cache with no live session behind it is kept before the sweep takes it. */
+export const AGENT_PREAMBLE_MAX_AGE_MS = 7 * 24 * 60 * 60 * 1000;
+
+/**
+ * Drop one session's preamble cache. Called when a session is deleted, which is the
+ * precise counterpart to seeding it at create: one file per claude session was being
+ * written and nothing ever removed them (236 leftovers measured on a working machine,
+ * the oldest three weeks old). Best-effort — a file that will not delete is litter,
+ * never a reason to fail a teardown.
+ */
+export async function removeAgentSessionPreamble(sessionId: string): Promise<void> {
+  await unlink(agentPreamblePath(sessionId)).catch(() => {});
+}
+
+/**
+ * Sweep preamble caches left by sessions that are gone: the delete path above covers
+ * an orderly teardown, and this covers everything else (a crash, a killed server, a
+ * session deleted by an older build, another instance's leftovers).
+ *
+ * ⚠️ Two guards, and both matter: a file whose session is in `keepSessionIds` is never
+ * touched however old it is, and everything else needs `maxAgeMs` of age on top. A live
+ * session's cache is load-bearing — remove it and the skill's two-line loader fails its
+ * version check mid-run — and the age floor is what keeps a session belonging to
+ * ANOTHER instance (whose ids this process cannot see) out of the blast radius. Losing
+ * one is degradation rather than breakage: the §0 fallback block rewrites it.
+ *
+ * Returns how many it removed. Best-effort throughout; a missing cache dir is 0.
+ */
+export async function pruneAgentSessionPreambles(
+  keepSessionIds: Iterable<string>,
+  maxAgeMs: number = AGENT_PREAMBLE_MAX_AGE_MS
+): Promise<number> {
+  const cacheDir = agentPreambleCacheDir();
+  const keep = new Set(keepSessionIds);
+  const cutoff = Date.now() - maxAgeMs;
+  let removed = 0;
+
+  let entries: string[];
+  try {
+    entries = await readdir(cacheDir);
+  } catch {
+    return 0;
+  }
+
+  for (const entry of entries) {
+    const sessionId = AGENT_PREAMBLE_FILE_PATTERN.exec(entry)?.[1];
+    if (!sessionId || keep.has(sessionId)) continue;
+    const path = join(cacheDir, entry);
+    try {
+      if ((await lstat(path)).mtimeMs > cutoff) continue;
+      await unlink(path);
+      removed++;
+    } catch {
+      /* best-effort — a vanished or unreadable file is not our problem */
+    }
+  }
+  return removed;
 }
 
 /**