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>

postal-service-addon/postal_service_RP/textures/terrain_texture.json

@@ -0,0 +1,14 @@
+{
+  "resource_pack_name": "postal_service_RP",
+  "texture_name": "atlas.terrain",
+  "padding": 8,
+  "num_mip_levels": 4,
+  "texture_data": {
+    "mailbox": {
+      "textures": "textures/blocks/mailbox"
+    },
+    "post_office": {
+      "textures": "textures/blocks/post_office"
+    }
+  }
+}