feat: route reply-quotes of past answers back to their source session

[loml13]

Motivation

A run's session is scoped to its chat (scope = chatId, or chatId:threadId
for topics). That means a conversation lives and dies in the chat it started
in: there's no way to pick a past answer back up from somewhere else. A common
want is "reply-quote something the bot told me earlier — even in another chat —
and keep talking to that conversation."

This PR adds an opt-in mechanism for exactly that: quote one of the bot's
earlier replies and the new run resumes the session that produced it.

Change

• New MessageRouteStore (src/session/message-routes.ts) — a small
  disk-backed ledger stored next to sessions.json (default
  <sessionsFile>.routes.json, mirroring the existing .catalog.json
  convention). It maps an outbound message_id → { scope, sessionId, cwd, ts }.
  Disk is the source of truth (each op reads the file fresh), so the bridge is
  not the only permitted writer: external notification tools that send
  messages "as" the bot through their own path can append their own entries to
  the same JSON object to make those messages reply-quote-routable too. This is
  a documented extension point (see the class doc-comment). Entries are bounded
  (LRU by ts, default 1000).

• Outbound registration — when a run sends its final reply, the bridge
  records that reply's message_id against the run's (possibly already-routed)
  scope, together with the session id / cwd the run used.

• Inbound routing — on intake, if a message reply-quotes a message_id
  that's in the ledger, the run's scope is overridden to the recorded scope.
  Because cwd resolution and session resume are both derived from the scope
  downstream, overriding the scope reconnects the run to the source
  conversation — the reply still lands in the chat the user wrote in, but the
  agent continues the quoted session.

Use case

1. Ask the bot something in chat A; it answers.
2. Later, in chat B (or a DM), reply-quote that answer and ask a follow-up.
3. The follow-up resumes chat A's session (same session id + cwd) instead of
   starting a blank one in chat B.

Compatibility

Fully backward compatible and opt-in:

• The feature is gated on startChannel({ messageRoutes }). Omit the store and
  nothing changes.
• Routing only ever fires on a ledger hit for a quoted message. Messages
  with no reply-quote, and quotes of messages that aren't in the ledger, take
  the exact same path as before — the non-quote path is untouched.
• The whole thing is best-effort: record never throws, lookup returns
  "no route" on a missing/corrupt ledger, and any error falls back to the
  normal per-chat scope. A broken ledger degrades to today's behavior, it can't
  break message handling.

Testing

• pnpm typecheck — clean.
• pnpm test — full suite green (adds 6 tests).
• tests/unit/session/message-routes.test.ts — record/lookup, external-writer
  entries surviving a subsequent bridge write, LRU eviction, and graceful
  degradation on a corrupt/missing/malformed ledger.
• tests/integration/bot/quote-route.test.ts — drives the real startChannel
  pipeline across two chats: a reply-quote of chat A's answer sent in chat B
  resumes chat A's session (sessionId + cwd), while a plain (non-quote)
  message in chat B starts its own session (no routing).

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