atomic-agents-stack: Dashboard HTTP server path traversal allows arbitrary file read

Summary

The optional dashboard HTTP server (`atomic_agents/dashboard/serve.py`) builds filesystem paths directly from the request path and serves them without a containment check. It is the only per-request untrusted-path site in the codebase that does not route through `_io.safe_resolve_under`. Literal `../` segments survive `urlparse` and `Path` joining, so a request can read files outside the intended `agents_root` (including via the static branch). **Impact:** arbitrary file read. The default bind is loopback, but `--host` is an operator-settable documented flag; binding `0.0.0.0` exposes this to the LAN. Even on loopback it is reachable via DNS-rebinding from a browser or SSRF from a co-located service. **Affected:** `dashboard/serve.py` (`DashboardHandler.do_GET` / `_serve_file`), all versions through 1.0.0. **Fix:** route every served path through `_io.safe_resolve_under` against the intended root and return 404 on `PathTraversalError`; reject `..`/separators early; optionally refuse a non-loopback `--host` unless an explicit auth/allow flag is set (matching `serve/_app.py`).