feat: add Slack thread mention-only routing

[RoySRose]

Summary

• add Slack thread-local mention-only flags controlled by natural-language on/off phrases
• block active-session, mentioned-thread, bot-reply, free-response, and mention-pattern wakeups while the flag is active
• prune flags by session idle TTL/default 24h and cap stored entries

Verification

• uv run --extra dev python -m pytest tests/gateway/test_slack_thread_mention_only.py tests/gateway/test_slack_mention.py -q -o 'addopts=' → 72 passed
• uv run --extra dev python -m pytest tests/gateway/test_slack.py tests/gateway/test_slack_thread_mention_only.py tests/gateway/test_slack_mention.py -q -o 'addopts=' → 288 passed
• uv run --extra dev ruff check plugins/platforms/slack/adapter.py tests/gateway/test_slack_thread_mention_only.py → All checks passed
• git diff --check -- plugins/platforms/slack/adapter.py tests/gateway/test_slack_thread_mention_only.py → clean

[alt-glitch]

This was generated by AI during triage.

Related: #24848 (open — routes plain replies from mentioned thread parents). Same Slack thread-mention-routing family but a different mechanism: this PR adds an opt-in per-thread "mention-only" silencing flag toggled by natural-language phrases, whereas #24848 fixes reply routing for already-mentioned parents. Prior strict_mention attempts (#12258, #14117, #44800) are closed. Not a duplicate — flagging the cluster so a maintainer can decide how these compose.

[RoySRose]

Closed: opened from a misinterpreted continuation. The Slack adapter change is not the active Waka approval task.