feat(bluesky): add Bluesky/AT Protocol as social source

Free, no-auth-required search via public.api.bsky.app.
Always-on like HN and Polymarket (no API key needed).

- New scripts/lib/bluesky.py: search + parse via AT Protocol
- BlueskyItem schema, normalization, scoring, deduplication
- Wired into orchestrator ThreadPoolExecutor with timeout config
- Rendering in compact, full, and JSON output modes
- 14 unit tests covering parsing, dates, relevance, edge cases
- --search=bluesky / --search=bsky for bluesky-only mode

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/lib/env.py

@@ -480,6 +480,14 @@ def is_hackernews_available() -> bool:
     return True
 
 
+def is_bluesky_available() -> bool:
+    """Check if Bluesky source is available.
+
+    Always returns True - AT Protocol search is free, no key needed.
+    """
+    return True
+
+
 def is_polymarket_available() -> bool:
     """Check if Polymarket source is available.