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_BP/blocks/private_chest.json

@@ -0,0 +1,27 @@
+{
+  "format_version": "1.21.0",
+  "minecraft:block": {
+    "description": {
+      "identifier": "silverlabs:private_chest",
+      "menu_category": {
+        "category": "items",
+        "group": "itemGroup.name.chest"
+      }
+    },
+    "components": {
+      "minecraft:destructible_by_mining": {
+        "seconds_to_destroy": 2.5
+      },
+      "minecraft:destructible_by_explosion": {
+        "explosion_resistance": 1200.0
+      },
+      "minecraft:map_color": "#B0B0B0",
+      "minecraft:material_instances": {
+        "*": {
+          "texture": "private_chest",
+          "render_method": "opaque"
+        }
+      }
+    }
+  }
+}