feat: add Slack thread mention-only routing

[hashbender]

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

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Mirror-of: NousResearch#56890
NousResearch#56890