Messaging users can now switch back to previously-named sessions:
- /resume My Project — resolves the title (with auto-lineage) and
restores that session's conversation history
- /resume (no args) — lists recent titled sessions to choose from
Adds SessionStore.switch_session() which ends the current session and
points the session entry at the target session ID so the old transcript
is loaded on the next message. Running agents are cleared on switch.
Completes the session naming feature from PR #720 for gateway users.
8 new tests covering: name resolution, lineage auto-latest, already-on-
session check, nonexistent names, agent cleanup, no-DB fallback, and
listing titled sessions.
