Add restricted AI worker access with deployment capabilities

- New module: modules/nixos/security/ai-worker-restricted.nix
  - Bind mount for infra repo access (RW)
  - Whitelisted sudo commands: nh, nixos-rebuild, nixpkgs-fmt, nix
  - Audit logging for infra changes
  - Documentation in README-ai-worker.md

- Updated users/ai-worker.nix:
  - Enable services.aiWorkerAccess
  - Lock password (SSH key only)
  - Security documentation comments

- Updated flake.nix:
  - Include new security module

SECURITY: AI must ask for user confirmation before running nh os switch

users/ai-worker.nix

@@ -9,6 +9,17 @@
     openssh.authorizedKeys.keys = [
       keys.users.ai-worker.main
     ];
+    # No password login - SSH key only
+    hashedPassword = "!";
   };
   users.groups.ai-worker = {};
+
+  # Enable restricted AI worker access with deployment capabilities
+  # SECURITY: ai-worker can only:
+  #   - Access /home/ai-worker/infra (bind-mounted to /home/gortium/infra)
+  #   - Run: nh, nixos-rebuild, nixpkgs-fmt, nix (via sudo, no password)
+  #   - Manage docker containers (via docker group)
+  #   - All changes to infra/ are logged via audit subsystem
+  # WORKFLOW: AI must ask for user confirmation before running nh os switch
+  services.aiWorkerAccess = true;
 }