feat(loop): headless sbx save/publish + hh-loop VM-library skill

Adds the autonomous /loop foundation: a headless `hack-house sbx save|publish`
subcommand so a non-TUI operator can persist a built VM to the host library
through the same canonical registry path the room UI uses (no schema drift).

- snapshot.rs: hoist register_saved_snapshot/publish_snapshot/oci_image_size out
  of app.rs into a shared module used by both the TUI and the new CLI.
- registry.rs: advisory cross-process lockfile (~/.hh/registry.lock, O_EXCL spin
  + stale-reclaim) around every read-modify-write, plus atomic temp+rename store,
  so concurrent /loop wave members can't clobber each other.
- main.rs: `Sbx { Save, Publish }` subcommand wired to the shared snapshot logic.
- skills/hh-loop: the loop doctrine — value rubric, adaptive 1-3 operator
  topology, visible-tmux-by-default run flow, --record logs/film, wave scaling,
  and safe `tmux -L hh-loop` teardown.

Proven end-to-end: built+verified a stdlib VM in a sandbox, headless
save+publish -> registry entry shareable:true with a portable tar.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -23,6 +23,10 @@ err.log
 
 # Sandbox save/load snapshots (large runtime tarballs, not source)
 /hh/hh-snapshots/
+# VM-library /loop artifacts — exported snapshot tars (repo-root, when `sbx`
+# is run from here) + per-run logs. Generated; the VMs live in the host registry.
+/hh-snapshots/
+/.loop-runs/
 
 # Out-of-tree experiments (not part of hack-house)
 /experiments/