feat(twitter): migrate from xreach to bird CLI

- Replace xreach CLI with bird (@steipete/bird) as Twitter/X backend
- bird uses AUTH_TOKEN/CT0 env vars (simpler than xreach's session.json)
- Accept both 'bird' and 'birdx' binary names
- Remove version detection logic (bird v0.8.0 is the baseline)
- Write credentials.env to ~/.config/bird/ for easy sourcing
- Keep xfetch session.json sync for backward compatibility
- Update SKILL.md commands: bird search/read/user-tweets/thread
- Update install/uninstall to use npm @steipete/bird
- All 52 tests pass

agent_reach/config.py

@@ -22,7 +22,7 @@ class Config:
     FEATURE_REQUIREMENTS = {
         "exa_search": ["exa_api_key"],
         "reddit_proxy": ["reddit_proxy"],
-        "twitter_xreach": ["twitter_auth_token", "twitter_ct0"],
+        "twitter_xreach": ["twitter_auth_token", "twitter_ct0"],  # legacy key name; used by bird CLI
         "groq_whisper": ["groq_api_key"],
         "github_token": ["github_token"],
     }