Closing in favor of the narrower salvage in #16193.

Your PR solved a real problem (thread auto-engagement in multi-agent scenarios), but the scope made it hard to merge as-is: +414/-136 across 10 files changed the default behavior in public channels and mixed in unrelated workspace-routing and allowed_users changes. The mention-gating piece is superseded by #16193's opt-in slack.strict_mention config (default off, backward-compatible) which reaches the same result for agents that need strict mode.

If you want the workspace routing / Socket Mode envelope team_id fix to land, please open a focused PR for just that piece — the code looked right, it was just coupled to the larger diff. Thanks for the contribution!