feat(slack): follow all replies in threads Cyrus is bound to (CYPACK-1267)

[cyrusagent]

Summary

Makes Cyrus feel "present" in a Slack thread it's been pulled into. After an initial @mention binds a thread, every subsequent reply in that thread is fed into the same agent session — no need to re-mention Cyrus each time. (CYPACK-1267)

This is the cyrus (runtime) half of the change; the cyrus-hosted half (webhook forwarding gate + Slack manifest) is in a paired PR.

What changed

• SlackEventTransport now accepts message events (not just app_mention). It applies cheap, conservative structural filters before doing any work:
  • drops the bot's own / other bots' messages (bot_id) — prevents reply loops
  • drops subtype events (edits, deletes, joins, bot_message, …)
  • drops non-threaded messages (only a threaded reply can belong to a bound thread)
  • de-dupes Slack's double-delivery — a message that mentions the bot arrives as both an app_mention and a message event; we collapse identical (channel, ts) pairs (10-min TTL) so the thread is prompted once.
• ChatPlatformAdapter gains an optional isSessionInitiatingEvent(event). ChatSessionHandler refuses to start a new session from a non-initiating event, so a plain reply only ever continues an already-bound thread — it never spins one up. (Defaults to "initiating" when the adapter omits the method, so Linear/GitHub behaviour is unchanged.)
• SlackChatAdapter marks app_mention as session-initiating and message as follow-up; SlackMessageTranslator now maps message → user prompt.
• Slack setup manifest (cyrus-setup-slack) subscribes to message.channels / message.groups / message.mpim / message.im (the *:history scopes were already present).

Design notes (SOLID)

• SRP — transport owns ingress + filtering + dedup; the adapter owns platform policy (isSessionInitiatingEvent); the handler owns session lifecycle.
• OCP/LSP/ISP — the new adapter method is optional and defaults to true, so existing adapters (and the test adapter) need no change.
• Works for both ingress paths: cyrus-hosted proxy forwarding and self-host direct Slack delivery (where cyrus is the only gate — hence the dedup + binding gate live here too).

Testing

• pnpm test:packages:run — all green (slack-event-transport 70, edge-worker 673, etc.). New tests cover: threaded message emitted, non-threaded/bot/subtype dropped, app_mention+message dedup, and the session-initiation gate.
• pnpm typecheck and pnpm lint clean.

Operational note

Existing Slack apps must add the four message.* bot events in Event Subscriptions for follow-ups to be delivered.