feat: Gitea Actions shell runners + flake-lock workflow

- Add modules/nixos/gitea-runner.nix: NixOS module for act_runner in
  shell mode, with options for enable, instanceURL, name, labels, and
  tokenFile (agenix secret path).
- Deploy gitea-runner to jeremiah, isaiah, zeke, and linode with
  appropriate labels. Agenix secret placeholders left with TODOs.
- Add .gitea/workflows/update-flake-lock.yaml: weekly workflow (Sunday
  midnight) that runs nix flake update and opens a PR if flake.lock
  changed, using GITEA_TOKEN secret for authentication.

Closes part of #15 (NixOS shell runners + flake-lock workflow).

hosts/unstable/zeke/default.nix

@@ -16,6 +16,8 @@
 
   age.secrets = {
     gitea-workerPassword.file = ../../../secrets/gitea/workerPassword.age;
+    # TODO: Greg add agenix secret at secrets/gitea-runner-zeke.age
+    # gitea-runner-zeke.file = ../../../secrets/gitea-runner-zeke.age;
   };
 
   boot.extraModulePackages = [ config.boot.kernelPackages.v4l2loopback ];
@@ -32,6 +34,12 @@
       enable = true;
       vbox = false;
     };
+    gitea-runner = {
+      enable = true;
+      labels = [ "self-hosted" "bare-metal" "host:zeke" ];
+      # TODO: Greg add agenix secret at secrets/gitea-runner-zeke.age
+      # tokenFile = config.age.secrets.gitea-runner-zeke.path;
+    };
     tailscale = {
       enable = true;
       tags = [ "home" ];