feat: migrate Twitter backend from bird CLI to xreach CLI

bird CLI (@steipete/bird) is deprecated and no longer maintained.
xreach CLI (xreach-cli on npm) is our maintained fork with:
- Fixed SearchTimeline (POST + updated query ID)
- Built-in proxy rotation support
- Additional features (DMs, notifications, lists)

Changes across 11 files:
- channels/twitter.py: detect xreach instead of bird/birdx
- cli.py: install/doctor/uninstall all reference xreach-cli
- SKILL.md: updated command examples (bird read → xreach tweet)
- guides/setup-twitter.md: rewritten for xreach
- docs/troubleshooting.md: updated proxy guidance
- README.md + README_en.md: all references updated
- config.py: twitter_bird → twitter_xreach
- core.py, mcp_server.py: comment updates

npm package: https://www.npmjs.com/package/xreach-cli
Source: https://github.com/Panniantong/xfetch

agent_reach/config.py

@@ -22,7 +22,7 @@ class Config:
     FEATURE_REQUIREMENTS = {
         "exa_search": ["exa_api_key"],
         "reddit_proxy": ["reddit_proxy"],
-        "twitter_bird": ["twitter_auth_token", "twitter_ct0"],
+        "twitter_xreach": ["twitter_auth_token", "twitter_ct0"],
         "groq_whisper": ["groq_api_key"],
         "github_token": ["github_token"],
     }