agents: enable generic multi-perspective code review agent

Configure git-repo-code-review-agent with use_default_subagents: true in
agents/agent_configs.txtpb so groupagent-grev-review uses the generic
gerrit_reviewer multi-perspective review framework.

Bug: 555706086
Change-Id: I079681b00e71cb886dc2343692d88b01d58a51ea
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/641904
Reviewed-by: Brian Gan <brgan@google.com>
Tested-by: Becky Siegel <beckysiegel@google.com>

agents/agent_configs.txtpb

@@ -24,3 +24,16 @@ configs {
   }
   automatic: false
 }
+
+# Default Multi-Perspective Code Review Agent
+configs {
+  id: "git-repo-code-review-agent"
+  display_name: "Code Review Agent"
+  description: "Default multi-perspective review agent for git-repo."
+  include_filters {
+    project: "git-repo"
+    path_regex: ".*"
+  }
+  use_default_subagents: true
+  tags: "code-review-agent"
+}