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).

nix/templates/agent-base.nix

@@ -6,13 +6,13 @@
     description = "hive-ag3nt harness";
     wantedBy = [ "multi-user.target" ];
     after = [ "network.target" ];
-    # `claude` for the turn loop + `bash` for claude's Bash tool. systemd
-    # units get a minimal PATH by default; entries in
-    # `environment.systemPackages` aren't on it.
-    path = [
-      pkgs.claude-code
-      pkgs.bashInteractive
-    ];
+    # systemd units get a minimal PATH by default and don't inherit
+    # `environment.systemPackages`. Pointing at `/run/current-system/sw`
+    # gives the harness (and any tools claude shells out to via Bash)
+    # access to everything declared in `systemPackages` — including
+    # anything an agent adds to its own `agent.nix` — without having to
+    # touch the service definition.
+    path = [ "/run/current-system/sw" ];
     environment.SHELL = "${pkgs.bashInteractive}/bin/bash";
     serviceConfig = {
       ExecStart = "${pkgs.hyperhive}/bin/hive-ag3nt serve";