plan prompt (#9943)

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codex-rs/core/templates/collaboration_mode/plan.md

@@ -28,6 +28,7 @@ You SHOULD ask many questions, but each question must:
 - materially change the spec/plan, OR
 - confirm/lock an assumption, OR
 - choose between meaningful tradeoffs.
+- not be answerable by non-mutating commands
 Batch questions (e.g., 4–10) per `request_user_input` call to keep momentum.
 
 ## Two kinds of unknowns (treat differently)