fix(mcp): migrate the last-read fallback on identity rename, purge, and channel rename/delete

[Lafnaps]

The #58 fallback (chat_send with no channel goes to the sender's last-read channel or job) is keyed by sender name and channel name, but none of the existing migration hooks move that state:

• migrate_identity() moves the read cursors but not _last_read_channel/_last_read_job_id. After claude becomes claude-1 (happens automatically when a second instance registers), its next bare chat_send falls back to #general instead of the channel it was just reading — the exact misfire Gemini switches chat channel in between sessions #58 was about.
• migrate_cursors_rename()/migrate_cursors_delete() leave stale channel values behind, so after a channel rename a fallback send posts into the old name and resurrects it as a ghost tab; after a delete it posts into a channel that no longer exists.
• purge_identity() leaks both maps, so an agent that later registers under the reused name inherits someone else's fallback.

The fix wires the maps into those four hooks, under the existing _last_read_lock: identity migrate rekeys both, purge pops both, channel rename rewrites matching channel values, channel delete resets them to general. (Job-id fallbacks are untouched by the channel hooks — job ids don't change when a channel is renamed.) +20/-2 in mcp_bridge.py.

Tests: four new cases in tests/test_channel_fallback.py following its existing state-map pattern, one per hook; the rename case also checks that other senders' fallbacks are left alone. python -m pytest -q tests/test_channel_fallback.py → 12 passed (8 on main). Full suite: 85 passed (81 on current main).