feat(postal): add postal service addon and bundle pending addon work

- New postal-service-addon: per-player mailboxes + post-office send block
  (ActionForm recipient picker, offline notification queue, chunk-load
  retry via tickingarea)
- Commit previously untracked private-chest, home-sign, keep-inventory
  addons and their docker-compose mounts
- Deploy workflow: add postal + previously unwired addons to path filter
  and checkout list; drop easter-egg from deployment
- enabled_packs.json: register postal UUIDs for Lyla + Mya

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

private-chest-addon/private_chest_RP/manifest.json

@@ -0,0 +1,17 @@
+{
+  "format_version": 2,
+  "header": {
+    "name": "Private Chests Resources",
+    "description": "Textures and lang for the silverlabs:private_chest block",
+    "uuid": "9a3f8d2e-7c5b-4e1a-b9d2-1f6e3c4a8b53",
+    "version": [1, 0, 0],
+    "min_engine_version": [1, 21, 0]
+  },
+  "modules": [
+    {
+      "type": "resources",
+      "uuid": "9a3f8d2e-7c5b-4e1a-b9d2-1f6e3c4a8b54",
+      "version": [1, 0, 1]
+    }
+  ]
+}