Slack bot: live-verify channel streaming, stop, and fallback behavior #1398

Summary

Verify the Slack bot's channel behavior against a real Slack workspace and
record the supported behavior in its documentation.

DM behavior has live evidence. Public and private channel handling, native
streaming, stop handling, and streaming fallback are implemented from Slack's
API contract but still carry an explicit live-verification caveat.

Scope

• Verify an @mention starts a session in a public channel thread.
• Verify an unmentioned reply in that thread continues the same session.
• Verify messages outside an activated thread remain ignored.
• Repeat the flow in a private channel with the checked-in manifest scopes.
• Verify chat.startStream, appendStream, and stopStream behavior.
• Verify agent_session_stopped cancels the exact active Mecatl run.
• Exercise the single-final-message fallback when Slack streaming fails.
• Record Slack configuration, observable evidence, and any required code or
  manifest corrections.

Completion criteria

• The documented public and private channel flows have live evidence.
• Streaming, cancellation, and fallback preserve one reply and one run per
  accepted Slack message.
• The README and design document state the verified behavior without the
  current caveats.

References

• Slack bot: implement once the SDK is ready #883 and sdk/typescript/examples/slack-bot/DESIGN.md.
• The Slack bot integration roadmap parent.

Fully or partially written by an AI agent.