refine thread tracking model to support parent-message tagging

[dembrane-sam-bot]

What is this change?

Refines the Slack thread-tracking model in the daemon to support parent-message tagging:

1. Parent Message Tagged (Case A): If the top-level parent message contains Sam's mention or is authored by the bot, Sam tracks this thread indefinitely and responds to untagged replies.
2. Parent Message Untagged (Case B): If the parent message is untagged but Sam is mentioned later in the thread, Sam responds to that specific mention but does not track subsequent replies (they are ignored unless a session is currently running on the thread, in which case they steer). Sam automatically appends a friendly reminder to tag Sam in subsequent messages.

What did Sam notice that led to this?

Jor requested a more refined thread tracking compromise to prevent thread pollution on threads that weren't originally initiated with Sam's involvement.

Tier

Tier 3 (modifying src/runtime/daemon.py and routing logic).

Confidence

100% confident. Added unit tests covering all new routing paths and verified the entire 600+ test suite is completely green.