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/jeremiah/default.nix

@@ -29,6 +29,8 @@ in
     gitea-oauthSecret = mk ../../../secrets/gitea/oauthSecret.age;
     gitea-webhookSecret = mk ../../../secrets/gitea/webhookSecret.age;
     gitea-workerPassword = mk ../../../secrets/gitea/workerPassword.age;
+    # TODO: Greg add agenix secret at secrets/gitea-runner-jeremiah.age
+    # gitea-runner-jeremiah.file = ../../../secrets/gitea-runner-jeremiah.age;
   };
 
   # Bootloader.
@@ -96,6 +98,12 @@ in
       threads = 3;
       qemu = true;
     };
+    gitea-runner = {
+      enable = true;
+      labels = [ "self-hosted" "bare-metal" "host:jeremiah" ];
+      # TODO: Greg add agenix secret at secrets/gitea-runner-jeremiah.age
+      # tokenFile = config.age.secrets.gitea-runner-jeremiah.path;
+    };
   };
 
   hardware = {