feat(identity): capability-layer positioning + skill rewrite, v1.5.0

SKILL.md (per the skill-writing guide):
- description rewritten trigger-first: 调研/全网调研/帮我调研/research lead
  the text (the owner's own phrasing "去全网调研" previously had NO matching
  trigger word — the exact undertrigger the guide warns about); platform
  keywords front-loaded against the 1536-char truncation
- standing rules added: probe `doctor --json` active_backend before acting
  on multi-backend platforms, announce which skill/backend is in use,
  follow the documented retry chains, compose multi-platform research
- quick commands updated to the new reality: bili-cli for bilibili search,
  Reddit/xiaohongshu moved to a "login-backed platforms" section
- SKILL_en.md: same description surgery + the actively harmful stale
  advice removed (anonymous reddit .json curl, yt-dlp for bilibili)

README / README_en:
- tagline gains the capability-layer subline: backends are chosen,
  installed and health-checked for us to swap — the user never notices
- "设计理念" reframed from scaffolding to capability layer (selection /
  install / doctor / routing); channels diagram now shows ordered backend
  lists; selection table split into primary + fallback columns with the
  live-test rationale per row
- platform table: honest rows for bilibili (bili-cli, yt-dlp retired),
  Reddit (no zero-config path), xiaohongshu (OpenCLI / mcp / legacy)

Version 1.5.0 across pyproject / __init__ / CLAUDE.md / test fixture.

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

tests/test_cli.py

@@ -148,7 +148,7 @@ class TestCheckUpdateRetry:
 
         sequence = [
             R(429, headers={"Retry-After": "3"}),
-            R(200, payload={"tag_name": "v1.4.2"}),
+            R(200, payload={"tag_name": "v1.5.0"}),
         ]
 
         with patch("requests.get", side_effect=sequence):