Replaces the single scoped `multica chat history --scope` read with two clean
noun-commands so the agent can navigate a channel with many threads (e.g. read
the specific thread a user referred to):
- `multica chat history` — the channel OVERVIEW: recent top-level messages, each
thread tagged with thread_id + reply_count + latest_reply (it does NOT expand
thread contents). Backed by GET /api/chat/history + slack.History.ChannelOverview
(conversations.history).
- `multica chat thread [id]` — read one thread: no id = the thread you're in,
an id = a specific thread IN THE SAME channel. Backed by GET /api/chat/thread +
slack.History.Thread (conversations.replies; DM falls back to history).
The channel stays server-pinned to the session; a thread id is only a
within-channel locator, so the security boundary (no cross-channel reads) is
unchanged. `--scope` is removed.
The prompt now teaches both commands and, via a new chat_in_thread signal
(derived from the binding: last_thread_id != last_message_id), tells the agent
which to start with — `chat history` for a top-level @mention, `chat thread` for
an in-thread one.
Tests: slack ChannelOverview/Thread (current/by-id/DM-fallback/no-binding/clamp),
handler both endpoints + auth, prompt top-level vs in-thread guidance.
Co-authored-by: J <[email protected]>
Co-authored-by: multica-agent <[email protected]>
