feat: subscribe to existing Slack channels (one session per thread)

[spacedragon]

Summary

Lets the Slack adapter subscribe to pre-existing Slack channels and run an AI agent against their messages — entirely within the adapter, no OpenACP core changes.

Model: one session per thread.

• An @mention (or, per-channel config, any top-level message) starts a new session bound to that message's thread; the agent works and replies inside that thread.
• A reply in an existing bot thread continues that session (Claude --resume, full context) — including after a restart.
• Tool-permission requests appear as buttons in the thread (built-in PermissionGate); a click resumes the agent. Free-text human replies in the thread continue the turn. No new HITL code.

Config (backward-compatible, defaults to []):

"subscribedChannels": [{ "channelId": "C0123ABCD", "trigger": "mention" }]

trigger: "mention" (default) or "all". Invite the bot to each channel.

How it works

• New pure module src/subscription-router.ts: classifySubscription (ignore / start / continue) + resolveThreadSession (binds a thread to a session, reusing the existing handleNewSession({ createThread: false }) primitive — no new Slack channel is created).
• event-router.ts runs the classifier before the unchanged legacy routing (owned-channel / DM / notification paths are byte-for-byte preserved).
• Output is threaded: all session-scoped posts add thread_ts when the session is subscription-bound; notification-channel posts are not threaded.

Safety

• Never archives a subscribed channel (deleteSessionThread + /openacp-archive guard on meta.threadTs).
• Per-thread permission cleanup so sibling threads sharing a channel aren't cross-wiped.
• Loop prevention: the bot's own messages are filtered before any routing.
• Restart-safe resume: thread→session detection consults the persisted store (getRecordByThread), not just live sessions.

Test Plan

• npm test — 139 passed, 1 skipped (new: subscription-router.test.ts 13 tests, plus event-router / text-buffer / activity-tracker / permission-handler additions).
• Manual: invite the bot to a test channel; @OpenACP ... at top level → a thread opens and the agent replies in-thread.
• Reply in the thread → agent continues with context.
• Trigger a tool needing permission → buttons appear in-thread; Allow → agent continues.
• Confirm the channel is never archived and sibling threads are unaffected.

Notes

• Design spec and implementation plan included under docs/superpowers/.
• A follow-up worth a separate ticket: a persisted record in error/cancelled state can dead-end a subscription thread (resume returns the "use /new" message, which isn't the subscription entry point). Pre-existing resume-design behavior, surfaced here.

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