feature: custom slack apps can read threads + send as user

[AggManik]

Custom Slack apps currently can’t reliably (a) read Slack threads or (b) send messages on the user’s behalf (user identity).

Current behavior

• No supported path for reading thread history for a given channel/thread
• No supported path for posting as the user (vs bot)

Expected behavior

• If a user installs a custom Slack app, core can read thread context (configurable scope)
• core can send a message either as bot or as the user (explicit mode), depending on what the user authorized

Suggested implementation notes

• Support both posting modes:
  • Bot token mode: chat.postMessage as the app
  • User token mode: chat.postMessage with a user token (or Slack Connect where applicable)
• OAuth + scopes:
  • Reading: channels:history, groups:history, im:history, mpim:history (depending on target), plus channels:read/groups:read
  • Thread expansion: use conversations.replies for thread_ts
  • Posting: chat:write (bot), chat:write:user (if using user token), plus users:read if needed
• Store token type and workspace metadata; enforce least-privilege and clear UX copy for what’s being authorized
• Add API surface to fetch a thread by (channel_id, thread_ts) and return messages in chronological order

Acceptance criteria

• Given (channel_id, thread_ts), core fetches the full thread messages reliably.
• core can post to a channel in bot mode.
• If user token authorized, core can post in user mode (and fails with a clear error when not authorized).
• Unit/integration tests cover: missing scopes, revoked token, and thread with > 100 replies (pagination).