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bird CLI (@steipete/bird) is deprecated and no longer maintained.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/channels/twitter.py

@@ -1,5 +1,5 @@
 # -*- coding: utf-8 -*-
-"""Twitter/X — check if bird CLI is available."""
+"""Twitter/X — check if xreach CLI is available."""
 
 import shutil
 import subprocess
@@ -9,7 +9,7 @@ from .base import Channel
 class TwitterChannel(Channel):
     name = "twitter"
     description = "Twitter/X 推文"
-    backends = ["bird CLI"]
+    backends = ["xreach CLI"]
     tier = 1
 
     def can_handle(self, url: str) -> bool:
@@ -18,21 +18,21 @@ class TwitterChannel(Channel):
         return "x.com" in d or "twitter.com" in d
 
     def check(self, config=None):
-        bird = shutil.which("bird") or shutil.which("birdx")
-        if not bird:
+        xreach = shutil.which("xreach")
+        if not xreach:
             return "warn", (
-                "bird CLI 未安装。搜索可通过 Exa 替代。安装：\n"
-                "  npm install -g @steipete/bird"
+                "xreach CLI 未安装。搜索可通过 Exa 替代。安装：\n"
+                "  npm install -g xreach-cli"
             )
         try:
             r = subprocess.run(
-                [bird, "whoami"], capture_output=True, text=True, timeout=10
+                [xreach, "auth", "check"], capture_output=True, text=True, timeout=10
             )
             if r.returncode == 0:
                 return "ok", "完整可用（读取、搜索推文）"
             return "warn", (
-                "bird CLI 已安装但未配置 Cookie。运行：\n"
+                "xreach CLI 已安装但未配置 Cookie。运行：\n"
                 "  agent-reach configure twitter-cookies \"auth_token=xxx; ct0=yyy\""
             )
         except Exception:
-            return "warn", "bird CLI 已安装但连接失败"
+            return "warn", "xreach CLI 已安装但连接失败"