Tasks: answer a Slack mention in the thread it came from

Four tasks, all in this repository. The regression test comes first so it is seen red; the state fields and the two code edits then make it green; docs close it.

Task list

[x] 1. Regression and integration tests (red). Write cli/tests/test_channels_reply_thread_integration.py (T2, T3, T4) and cli/tests/test_channels_reply_thread.py (T1). Run them on the unchanged code and record the red result.
- Requirements: R1–R3, abuse cases 1–2
- Test: T1–T4
- Depends on: —
[x] 2. State fields and is_progress. Add replyChannel / replyThread to _delivery_record; make room_policy.is_progress public (keep the private alias's callers working).
- Requirements: data model, R2.3, migration
- Test: T1
- Depends on: 1
[x] 3. Note on inbound, use on outbound. _note_reply_thread in inbound.process_reply; _reply_thread and the thread_ts / PostResult.thread change in SlackBotChannel.post.
- Requirements: R1.1–R1.5, R2.1–R2.4, R3.1–R3.2
- Test: T2, T3, T4
- Depends on: 2
[x] 4. Docs. Amend the issue-378 bullet in docs/capabilities/channels.md and add its history row; one paragraph in docs/guide/slack.md; evidence/documentation.md.
- Requirements: capability-docs gate
- Test: T5 (markdownlint)
- Depends on: 3

Dependency graph (DAG)

Checkpoints

After task 1: the new tests fail for the right reason (top-level post).
After task 3: cd cli && uv run python -m pytest -q is green.
After task 4: uv run pre-commit run --all-files is green.