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_BP/recipes/mailbox.json

@@ -0,0 +1,18 @@
+{
+  "format_version": "1.21.0",
+  "minecraft:recipe_shapeless": {
+    "description": {
+      "identifier": "silverlabs:mailbox_recipe"
+    },
+    "tags": ["crafting_table"],
+    "unlock": { "context": "AlwaysUnlocked" },
+    "ingredients": [
+      { "item": "minecraft:chest" },
+      { "item": "minecraft:gold_ingot" }
+    ],
+    "result": {
+      "item": "silverlabs:mailbox",
+      "count": 1
+    }
+  }
+}