programs.git.enable + harness PATH tracks systemPackages

- harness-base.nix: switch to programs.git for declarative gitconfig.
- agent + manager service path = /run/current-system/sw → agents pick up
  new packages from their own agent.nix without harness edits.
- generated applied/<name>/flake.nix overrides programs.git.config.user
  (no more raw etc.gitconfig collision).

hive-c0re/src/lifecycle.rs

@@ -243,13 +243,10 @@ pub async fn setup_applied(
         modules = [
           ./agent.nix
           {{
-            environment.etc."gitconfig".text = ''
-              [user]
-                name = {name}
-                email = {name}@hyperhive
-              [init]
-                defaultBranch = main
-            '';
+            programs.git.config.user = {{
+              name = "{name}";
+              email = "{name}@hyperhive";
+            }};
             systemd.services.{service}.environment = {{
               HIVE_PORT = "{port}";
               HIVE_LABEL = "{name}";