fix: doctor status must be honest — show real capability per channel

Before: everything under tier 0 showed ✅ with vague descriptions
After: each channel shows its actual capability and limitations

Changes:
- GitHub: 'Public repos only. Set github_token for private repos'
- Twitter: shows 'Full access' vs 'Read-only' depending on birdx
- Bilibili: ⚠️ warning on servers about potential IP blocks
- XiaoHongShu: friendly message when no cookie (was showing Jina 451)
- Doctor format: tier 0 items now show detailed status messages, not just ✅
- README: platform table updated to match reality (removed exaggerated claims)
- README: doctor example updated to show new honest format

agent_eyes/channels/github.py

@@ -13,7 +13,7 @@ from typing import List
 
 class GitHubChannel(Channel):
     name = "github"
-    description = "GitHub repos, issues, PRs, code"
+    description = "GitHub repos and code"
     backends = ["GitHub API"]
     tier = 0
 
@@ -26,6 +26,12 @@ class GitHubChannel(Channel):
             h["Authorization"] = f"Bearer {token}"
         return h
 
+    def check(self, config=None):
+        token = config.get("github_token") if config else None
+        if token:
+            return "ok", "Full access (authenticated)"
+        return "ok", "Public repos only. Set github_token for private repos + higher rate limits"
+
     def can_handle(self, url: str) -> bool:
         domain = urlparse(url).netloc.lower()
         return "github.com" in domain