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/public/README.md

@@ -0,0 +1,8 @@
+本目录用于 **公共 skills**。
+
+约定：
+- 路径：`oclaw/skills/_workspace/public/<skill_name>/SKILL.md`
+- 放在这里的 skill **默认对所有人可用**，不需要在 Admin 里做角色绑定。
+
+注意：
+- 仍会受 skill 本身的 `disable_model_invocation`、租户禁用列表等策略影响。