Adds RedditCommentStrategy to app/services/platforms/reddit_comment.py,
resolving thread_id via thread_url_override or _find_sticky title search,
falling back to reconstructed URL when client.comment() returns empty string.
Registers the strategy under "reddit_comment" in the platform _REGISTRY.
7 new tests confirm all execution paths: url override, title pattern lookup,
not-found error, missing-extra error, empty-URL reconstruction, dupe guard,
and registry presence. Full suite: 34/34 passing.
