add hive-forge module: private forgejo for agents

new `services.hive-forge.enable` (off by default) wraps
`services.forgejo` with hyperhive-friendly defaults: sqlite (no
extra db service), built-in ssh on 2222 so it doesn't fight the
host's openssh, http on 3000 (outside hyperhive's 7000/8000/8100-8999
ranges), registration off (operator seeds agent users), private
repos by default. exported as `nixosModules.hive-forge` — operator
imports it on the host alongside hive-c0re. container-side wiring
(MCP tools or a bind-mounted token) is deferred; containers already
share the host netns so they can reach http://localhost:3000 today.

nix/modules/hive-forge.nix

@@ -0,0 +1,116 @@
+{
+  pkgs,
+  lib,
+  config,
+  ...
+}:
+let
+  cfg = config.services.hive-forge;
+in
+{
+  # Thin wrapper around `services.forgejo` with hyperhive-friendly
+  # defaults: sqlite (no extra service to manage), built-in SSH on a
+  # non-22 port so it doesn't fight the host's sshd, registration off
+  # (agents get accounts seeded out of band), and ports opened in the
+  # firewall.
+  #
+  # Forge wiring into the agent containers is intentionally out of
+  # scope here — containers already share the host network namespace,
+  # so once this module is enabled an agent can reach the forge at
+  # http://localhost:<httpPort> without any extra bind mount. The MCP
+  # tool surface (open PR, list repos, etc.) lives in a separate
+  # follow-up that the operator opts into per agent.
+
+  options.services.hive-forge = {
+    enable = lib.mkEnableOption "hive-forge — private Forgejo instance for hyperhive agents";
+
+    httpPort = lib.mkOption {
+      type = lib.types.port;
+      default = 3000;
+      description = ''
+        TCP port the forge serves HTTP on. Default 3000 sits outside
+        hyperhive's claimed ranges (dashboard 7000, manager 8000,
+        sub-agents 8100..8999) so they don't collide.
+      '';
+    };
+
+    sshPort = lib.mkOption {
+      type = lib.types.port;
+      default = 2222;
+      description = ''
+        TCP port the forge's built-in SSH server listens on. Kept off
+        22 so it doesn't clash with the host's openssh. Agents push
+        with `ssh -p <sshPort> git@<domain>:<owner>/<repo>.git`.
+      '';
+    };
+
+    domain = lib.mkOption {
+      type = lib.types.str;
+      default = "localhost";
+      example = "forge.internal";
+      description = ''
+        Hostname agents and operator dial the forge by. `localhost` is
+        fine for a single-host setup since containers share the host
+        netns; set a real hostname when you want clones from outside
+        the host to look canonical.
+      '';
+    };
+
+    openFirewall = lib.mkOption {
+      type = lib.types.bool;
+      default = true;
+      description = ''
+        Open `httpPort` + `sshPort` in the host firewall. Off when the
+        forge should only be reachable from inside the host (the
+        forge listens on all interfaces either way; this just gates
+        external access).
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    services.forgejo = {
+      enable = true;
+      database.type = "sqlite3";
+      lfs.enable = true;
+      settings = {
+        server = {
+          DOMAIN = cfg.domain;
+          ROOT_URL = "http://${cfg.domain}:${toString cfg.httpPort}/";
+          HTTP_PORT = cfg.httpPort;
+          START_SSH_SERVER = true;
+          SSH_PORT = cfg.sshPort;
+          SSH_LISTEN_PORT = cfg.sshPort;
+          BUILTIN_SSH_SERVER_USER = "git";
+          DISABLE_SSH = false;
+        };
+        # Registration off — operator creates agent users via
+        # `forgejo admin user create` (or the API once seeded).
+        # Agents that need access get a dedicated account + token.
+        service = {
+          DISABLE_REGISTRATION = true;
+          REQUIRE_SIGNIN_VIEW = false;
+        };
+        repository = {
+          DEFAULT_BRANCH = "main";
+          DEFAULT_PRIVATE = "private";
+        };
+        # Quiet logs at idle; bump when debugging.
+        log.LEVEL = "Warn";
+      };
+    };
+
+    networking.firewall = lib.mkIf cfg.openFirewall {
+      allowedTCPPorts = [
+        cfg.httpPort
+        cfg.sshPort
+      ];
+    };
+
+    # Convenience: drop the forgejo CLI on the host PATH so the
+    # operator can `forgejo admin user create …` without hunting for
+    # the wrapped binary. The forgejo service runs as its own user;
+    # admin commands need `sudo -u forgejo`.
+    environment.systemPackages = [ pkgs.forgejo ];
+  };
+}