wip(#1970): add hyperhive.githubAccount nix option (single-account, nullable)

nix/templates/harness-base.nix

@@ -624,6 +624,76 @@ in
     '';
   };
 
+  options.hyperhive.githubAccount = lib.mkOption {
+    type = lib.types.nullOr (
+      lib.types.submodule {
+        options = {
+          username = lib.mkOption {
+            type = lib.types.str;
+            example = "the-sword-above";
+            description = ''
+              GitHub login the agent acts as. Used as the username for
+              `git push` over HTTPS and shown to the agent via the
+              `HIVE_GITHUB_USER` environment variable. Should be a
+              dedicated bot account, never a human's.
+            '';
+          };
+          tokenFile = lib.mkOption {
+            type = lib.types.str;
+            example = "/agents/damocles/state/github-token";
+            description = ''
+              Path to the file holding this account's personal access
+              token (PAT). The token *value* is never in nix --- the
+              provisioner (dashboard credentials tab, or `hivectl`)
+              writes an operator-supplied PAT here (0600, agent-owned),
+              the same contract as `matrixAccounts.<name>.tokenFile`.
+              The `gh` wrapper and the git credential helper read the
+              token from this path at invocation time, so a PAT pasted
+              mid-session takes effect with no rebuild. `gh` / `git push`
+              simply fail unauthenticated until the file exists.
+
+              Keep the PAT minimally scoped (only the repos/scopes the
+              agent's workflow needs): the agent has passwordless sudo,
+              so a compromised agent can act as the account within the
+              token's scopes --- scope is the real blast-radius limiter.
+            '';
+          };
+          host = lib.mkOption {
+            type = lib.types.str;
+            default = "github.com";
+            example = "github.example.com";
+            description = ''
+              GitHub host. Defaults to `github.com`; set it for a GitHub
+              Enterprise instance. Drives both the `gh` API host
+              (`GH_HOST`) and the git credential helper's URL match.
+            '';
+          };
+        };
+      }
+    );
+    default = null;
+    example = lib.literalExpression ''
+      {
+        username = "the-sword-above";
+        tokenFile = "/agents/damocles/state/github-token";
+      }
+    '';
+    description = ''
+      Give the agent a managed GitHub account: a `gh` CLI wrapper and a
+      git credential helper, both authenticated from an operator-supplied
+      PAT, so the agent can run `gh` API calls and `git push` to GitHub as
+      the configured login without any manual `gh auth login` dance.
+
+      `null` (the default) leaves GitHub off entirely --- no `gh` wrapper,
+      no credential helper, no env. When set, the token is supplied out of
+      band (dashboard credentials tab / `hivectl`) into `tokenFile`; nix
+      only carries the login + host, never the secret.
+
+      Single account per agent by design (unlike `matrixAccounts`, which is
+      multi-account): the GitHub workflow is "this agent is this one bot".
+    '';
+  };
+
   options.hyperhive.frontend.dist = lib.mkOption {
     type = lib.types.package;
     default = pkgs.hyperhive-frontend;