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/core.py

@@ -3,7 +3,7 @@
 AgentReach — installer, doctor, and configuration tool.
 
 Agent Reach helps AI agents install and configure upstream platform tools
-(xreach CLI, yt-dlp, mcporter, gh CLI, etc.). After installation, agents
+(bird CLI, yt-dlp, mcporter, gh CLI, etc.). After installation, agents
 call the upstream tools directly — no wrapper layer needed.
 
 Usage: