fix: use reason_subject (the actual post) not uri (the like record)
notification.uri for likes/reposts is the engagement record itself,
not the post that was liked. get_posts on a like URI returns empty,
so thread context was never fetched. reason_subject has the post URI.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>