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/skill/SKILL.md

@@ -39,13 +39,13 @@ mcporter call 'exa.web_search_exa(query: "query", numResults: 5)'
 mcporter call 'exa.get_code_context_exa(query: "code question", tokensNum: 3000)'
 ```
 
-## Twitter/X (xreach)
+## Twitter/X (bird)
 
 ```bash
-xreach search "query" -n 10 --json          # search
-xreach tweet URL_OR_ID --json                # read tweet (supports /status/ and /article/ URLs)
-xreach tweets @username -n 20 --json         # user timeline
-xreach thread URL_OR_ID --json               # full thread
+bird search "query" -n 10                  # search
+bird read URL_OR_ID                        # read tweet (supports /status/ and /article/ URLs)
+bird user-tweets @username -n 20           # user timeline
+bird thread URL_OR_ID                      # full thread
 ```
 
 ## YouTube (yt-dlp)