refactor: move bundled skills tree to repo root skills/

- Relocate runtime/skills -> skills/; runtime_skills_root() prefers oclaw/skills
  with fallback to legacy runtime/skills for unmigrated trees.
- session-bootstrap hook handler resolves repo root via PROJECT_ROOT.
- Update architecture/migration docs and in-tree skill references.

Co-authored-by: Cursor <cursoragent@cursor.com>

skills/_workspace/generalist/multi-search-engine/metadata.json

@@ -0,0 +1,7 @@
+{
+  "name": "multi-search-engine",
+  "version": "2.1.0",
+  "description": "Multi search engine with 16 engines (7 CN + 9 Global). Supports advanced operators, time filters, privacy engines.",
+  "engines": 16,
+  "requires_api_key": false
+}