feat(vagrant): backpack as inventory container — end-to-end pickup/drop demo

- Add container={kind="list"} to the backpack template so inventory.add
  accepts it as a valid list-container
- Remove old backpack proximity-trigger (print-only callback, pre-B.5);
  the rock's register_pickup is now the sole interaction trigger
- Emit unconditional inventory inspection trace after every add/remove:
  backpack count + item reg-ids for user-facing demo feedback
- Add CI-gated tree-check traces: tree_check_after_pickup=ok and
  tree_check_after_drop=ok verify composition-tree child placement
- Bump version 0.7.3 -> 0.8.0 across init.lua header, manifest.module,
  and README
- README: rewrite Controls section into full demo walkthrough; add Demo
  Walkthrough section; update Demonstrates + Interactions sections;
  add v0.8.0 CHANGELOG entry

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

manifest.module

@@ -1,6 +1,6 @@
 {
     "id": "vagrant-skeleton",
-    "version": "0.7.3",
+    "version": "0.8.0",
     "kind": "module",
     "api": "^0.1",
     "ci_frames": 60,