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/linode/default.nix

@@ -23,6 +23,11 @@
     pgloader
   ];
 
+  age.secrets = {
+    # TODO: Greg add agenix secret at secrets/gitea-runner-linode.age
+    # gitea-runner-linode.file = ../../../secrets/gitea-runner-linode.age;
+  };
+
   greg = {
     home = false;
     linode.enable = true;
@@ -36,6 +41,12 @@
       ssl = true;
     };
     tailscale.enable = true;
+    gitea-runner = {
+      enable = true;
+      labels = [ "self-hosted" "host:linode" ];
+      # TODO: Greg add agenix secret at secrets/gitea-runner-linode.age
+      # tokenFile = config.age.secrets.gitea-runner-linode.path;
+    };
   };
 
   networking = {