Document external agent skills root

docs/reference/repo-tree.md

@@ -6,6 +6,7 @@
   - package.json / bun.lock (Root Bun tooling for CLI checks)
   - .gitignore
   - .agents/skills/ (Repo-owned UniDesk agent skills. Skill directories named `unidesk-*` live here as the Git-backed source of truth; `~/.agents/skills` is reserved for non-UniDesk shared/external skills or local runtime copies.)
+  - ~/.agents/skills/ (External shared skill root when present. On UniDesk operator hosts this may be a separate Git clone of `https://github.com/pikasTech/agent_skills` named `skills`; validate structure and size with `bun scripts/cli.ts health --skills-dir ~/.agents/skills --threshold-bytes 10240`.)
   - reference -> docs/reference (Compatibility symlink for older references)
   - scripts/ (Unified CLI and implementation modules)
     - cli.ts (Single Bun CLI entry)