fix(coding-agent): make subagent deletion user-requested in the system prompt

packages/coding-agent/.changes/prompt-subagent-deletion-user-only.md

@@ -0,0 +1 @@
+- The system prompt no longer tells the model to delete finished subagents by default; deletion is now described as user-requested only, so idle children stay available for follow-ups.

packages/coding-agent/src/core/prompts/rlm.ts

@@ -169,7 +169,7 @@ export function buildRlmPrompt(options: RlmPromptOptions): string {
 			parts.push("Inspect files a child wrote when you need to collect its work without an observation capability.");
 		}
 		parts.push(
-			"Spawn independent children in separate calls and end your turn instead of awaiting completion. Multiple replies may arrive over multiple turns. Delete a direct child explicitly with `await rlm.delete_subagent(child)` when it is no longer needed.",
+			"Spawn independent children in separate calls and end your turn instead of awaiting completion. Multiple replies may arrive over multiple turns. You can delete a direct child explicitly with `await rlm.delete_subagent(child)`. Only do this if and when a user asks for it.",
 		);
 	}