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

tests/test_doctor.py

@@ -16,7 +16,7 @@ class TestDoctor:
         results = check_all(tmp_config)
         assert results["web"]["status"] == "ok"
         assert results["github"]["status"] == "ok"
-        assert results["bilibili"]["status"] == "ok"
+        assert results["bilibili"]["status"] in ("ok", "warn")  # warn on servers
         assert results["rss"]["status"] == "ok"
 
     def test_exa_off_without_key(self, tmp_config):