homelab: add deploy.enable option with assertion

- Add homelab.deploy.enable option (requires vault.enable)
- Create shared homelab-deploy Vault policy for all hosts
- Enable homelab.deploy on all vault-enabled hosts

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

modules/homelab/default.nix

@@ -1,6 +1,7 @@
 { ... }:
 {
   imports = [
+    ./deploy.nix
     ./dns.nix
     ./host.nix
     ./monitoring.nix