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>

keep-inventory-addon/keep_inventory_BP/scripts/main.js

@@ -0,0 +1,17 @@
+import { world, system } from "@minecraft/server";
+
+function assertKeepInventory() {
+  try {
+    world.getDimension("overworld").runCommand("gamerule keepInventory true");
+  } catch (e) {
+    // Non-fatal — chunk-load races, etc. The next interval will retry.
+  }
+}
+
+system.run(() => {
+  assertKeepInventory();
+  world.sendMessage("§a[Keep Inventory] §7keepInventory enforced — items stay on death.");
+});
+
+// Re-assert every 10 minutes so anything that toggles it off (manual op, world reset, etc.) gets corrected.
+system.runInterval(assertKeepInventory, 12000);