Fix Slack current-thread channel inference

[decofe]

Summary

• Cross-check API Slack session context against the bound tool thread key before inferring upload destinations
• Fall back to the tool thread key when API context is stale or mismatched
• Add regression coverage for Slack channel mention destinations and stale API context

Tests

• PYTHONPATH=/home/agent/branches/paradigmxyz/centaur/tools/productivity:/home/agent/branches/paradigmxyz/centaur uv run --no-project --with pytest --with slack-sdk --with structlog pytest /home/agent/branches/paradigmxyz/centaur/tools/productivity/slack/tests/test_client.py

Prompted by: @gakonst