[codex] Bias group chat prompts toward subagent delegation (#74046)

Bias group-chat prompt composition toward using subagents for tool-heavy work, keeping maintainer-channel responsiveness higher.\n\nValidated locally with focused prompt/auto-reply tests before opening the PR.

src/agents/prompt-composition.test.ts

@@ -63,10 +63,12 @@ describe("prompt composition invariants", () => {
     const eventTurn = getTurn(groupScenario!, "t3");
 
     expect(first.systemPrompt).toContain("You are in a Slack group chat.");
+    expect(first.systemPrompt).toContain("prefer delegating bounded side investigations early");
     expect(first.systemPrompt).toContain("Activation: trigger-only");
     expect(first.systemPrompt).toContain('reply with exactly "NO_REPLY"');
     expect(first.systemPrompt).not.toContain("## Silent Replies");
     expect(steady.systemPrompt).toContain("You are in a Slack group chat.");
+    expect(steady.systemPrompt).toContain("prefer delegating bounded side investigations early");
     expect(steady.systemPrompt).toContain('reply with exactly "NO_REPLY"');
     expect(steady.systemPrompt).not.toContain("## Silent Replies");
     expect(steady.systemPrompt).not.toContain("Activation: trigger-only");

src/auto-reply/reply/groups.test.ts

@@ -31,6 +31,8 @@ describe("group runtime loading", () => {
       "You are in a WhatsApp group chat. Your replies are automatically sent to this group chat. Do not use the message tool to send to this same group - just reply normally.",
     );
     expect(groupChatContext).toContain("Minimize empty lines and use normal chat conventions");
+    expect(groupChatContext).toContain("prefer delegating bounded side investigations early");
+    expect(groupChatContext).toContain("Keep the critical path local");
     expect(groupChatContext).toContain('reply with exactly "NO_REPLY"');
     const toolOnlyContext = groups.buildGroupChatContext({
       sessionCtx: { ChatType: "group", Provider: "discord" },

src/auto-reply/reply/groups.ts

@@ -245,6 +245,9 @@ export function buildGroupChatContext(params: {
   lines.push(
     "Write like a human. Avoid Markdown tables. Minimize empty lines and use normal chat conventions, not document-style spacing. Don't type literal \\n sequences; use real line breaks sparingly.",
   );
+  lines.push(
+    "When subagent or session-spawn tools are available and a directly requested group-chat task will require several tool calls, prefer delegating bounded side investigations early so the channel gets a responsive path forward. Keep the critical path local, avoid subagents for simple one-step work, and only surface concise group-visible updates when they add value.",
+  );
   const canUseSilentReply =
     !messageToolOnly &&
     params.silentToken &&