fix(slack): keep status line on legacy assistant.threads.setStatus

[palol]

Problem

Since the Agent Sessions API routing landed (commit a5522f69c0), the Slack
"thinking" status line silently stopped rendering. On slack-sdk 3.44+, the adapter
prefers agents.sessions.setStatus, but that endpoint's status parameter is a
closed enum (active|processing|suspended|closed) with no free-text field.

Hermes sends free text ("is thinking...", "still working… (2m03s)") and "" to
clear, so every call returns invalid_arguments: must be a valid enum value [json-pointer:/status], which _set_thread_status swallows at logger.debug.

Repro (live bot token):

• agents.sessions.setStatus(status="is thinking...") -> invalid_arguments
• agents.sessions.setStatus(status="") -> invalid_arguments (the clear path)
• agents.sessions.setStatus(status="processing") -> ok
• assistant.threads.setStatus(status="is thinking...") -> ok:true (legacy accepts free text)

Fix

Keep the status line on the legacy assistant.threads.setStatus (free-text
capable, supported until the Feb 2027 assistant-view deprecation). Route only the
title through agents.sessions.rename.

• plugins/platforms/slack/adapter.py: _session_status_method now always returns
  client.assistant_threads_setStatus, with a comment stating the WHY (enum-only API).
• tests/gateway/test_slack.py: the two Agent-Sessions status tests now assert the
  status stays on the legacy method even when the SDK exposes agent sessions (red on
  base, green with the fix). Title-via-rename tests unchanged.
• website/docs/user-guide/messaging/slack.md: corrected the two places that claimed
  the status call routes through agents.sessions.setStatus.

Test plan

scripts/run_tests.sh tests/gateway/test_slack.py — 244 passed, 0 failed, 1 skipped.

[alt-glitch]

This was generated by AI during triage.

Duplicate of #110391. Both PRs unconditionally keep free-text status on assistant.threads.setStatus and change the same adapter/tests; #110391 is the earlier PR by the original issue reporter. This PR's docs correction can be folded into that canonical PR.