fix: apply the review items for #690 at merge

Boot prune keeps the inboxes of sessions on the reboot-restore offer (their records are already pruned and a rebuild reuses the id), with a case in test/agent-inbox-server-wiring.test.ts.
docs/wiki: post, inbox and ack in the codeman agent command block of Driving-Codeman-From-An-Agent, and "Keep agent inbox across restarts" in Settings-Reference.
codeman agent inbox prints message text through terminalSafeText (ANSI stripped, other C0, DEL and C1 controls shown as \xHH); --json stays as stored.
Shutdown flushes the inbox snapshot again after app.close(), for a post that lands after the first flush.
POST /api/cleanup-state drops the inboxes of the records it removes.
InboxPostSchema derives its fence from MAX_MESSAGE_BYTES instead of the literal.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/api-reference.md

@@ -668,7 +668,8 @@ detached. By default inboxes live in memory and a server restart empties them.
 The synced setting `agentInboxPersistEnabled` (default off; Settings → Cross-session features)
 writes them to `agent-inbox.json` in the data dir (mode `0600`, tmp file +
 rename) and restores them at boot. Inboxes of sessions that did not come back
-are pruned only after the session restore succeeded. Switching the setting off
+are pruned only after the session restore succeeded; a session on the
+reboot-restore offer keeps its inbox, so a rebuild finds its mail. Switching the setting off
 deletes the file and keeps what is in memory. The file is never read by
 `/api/search`, and the file viewer refuses it.