fetch thread context for engagement notifications (likes/reposts)
engagement entries were built with empty thread_context, so when the
owner liked a post in a thread about following someone, phi had no way
to connect the like to the pending action. now likes/reposts resolve
thread refs and fetch the full conversation, same as mentions/replies.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>