feat(bilibili,twitter): yt-dlp exits bilibili; OpenCLI joins both

bilibili:
- live-verified 2026-06: bilibili 412-blocks yt-dlp in every
  configuration (latest version, direct, proxied, warmed cookies) while
  bili-cli works fine without login — so yt-dlp no longer serves this
  channel (it remains the YouTube backend)
- backends = [bili-cli, OpenCLI, B站搜索 API]: bili-cli covers
  search/hot/rank/video-detail/audio, OpenCLI adds subtitles through the
  browser session, the search API is the zero-dependency fallback
- when a broken candidate is bypassed by a working fallback, its
  reinstall prescription is appended to the winning message instead of
  being swallowed
- skill docs (social.md + video.md): "do NOT use yt-dlp for bilibili"
  warning, bili-cli/OpenCLI command groups, curl fallback recipe,
  B站 audio transcription path via `bili audio` + agent-reach transcribe

twitter:
- OpenCLI joins as the middle candidate [twitter-cli, OpenCLI, bird]
- social.md: explicit 4-step search retry chain (retry → upgrade →
  OpenCLI → stable-command detour)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

agent_reach/channels/twitter.py

@@ -8,7 +8,7 @@ from agent_reach.probe import probe_command
 class TwitterChannel(Channel):
     name = "twitter"
     description = "Twitter/X 推文"
-    backends = ["twitter-cli", "bird CLI (legacy)"]
+    backends = ["twitter-cli", "OpenCLI", "bird CLI (legacy)"]
     tier = 1
 
     def can_handle(self, url: str) -> bool:
@@ -24,6 +24,8 @@ class TwitterChannel(Channel):
         for backend in self.ordered_backends(config):
             if backend == "twitter-cli":
                 result = self._check_twitter_cli()
+            elif backend == "OpenCLI":
+                result = self._check_opencli()
             elif backend == "bird CLI (legacy)":
                 result = self._check_bird()
             else:
@@ -84,6 +86,22 @@ class TwitterChannel(Channel):
             "  twitter -v status 查看详细信息"
         )
 
+    def _check_opencli(self):
+        """OpenCLI candidate. None = not installed."""
+        from agent_reach.backends import opencli_status
+
+        st = opencli_status()
+        if not st.installed:
+            return None
+        if st.broken:
+            return "error", st.hint
+        if st.ready:
+            return "ok", (
+                "OpenCLI 可用（复用浏览器登录态）。用法："
+                "opencli twitter search/article/user-posts -f yaml"
+            )
+        return "warn", st.hint
+
     def _check_bird(self):
         """探测 bird/birdx（legacy 回退）。返回 None 表示均未安装，否则返回 (status, message)。"""
         last_failure = None