Retain user instructions in guardian thread context (#42762)

## Why

Guardian review needs the original user instructions alongside verified answers, even after compaction, resume, or rollback. A summarized or legacy checkpoint cannot establish a complete instruction history.

## What changed

- Capture text user messages in retained context when `guardian_thread_context` is enabled, preserving their message identity through compaction and replay.
- Keep retained instructions and verified answers in arrival order, with independent size and completeness tracking for each family.
- Roll back retained evidence at the exact user-message boundary, including steers that share a turn ID, and apply the same semantics during rollout migration.
- Treat checkpoints without retained user messages as incomplete while preserving their verified answers.

## Testing

- Cover capture, compaction, resume, rollback, child forks, legacy checkpoints, storage bounds, and rollout migration.

GitOrigin-RevId: 72706731b54e5ac8645db61b446d2401afb42235

codex-rs/guardian-context/src/verified_answers.rs

@@ -17,7 +17,7 @@ pub struct RenderedVerifiedAnswers {
 
 /// Renders each response with its original source roles and the existing approximate per-answer budget.
 pub fn render_verified_answers(context: &RetainedContext) -> RenderedVerifiedAnswers {
-    let mut complete = context.is_complete();
+    let mut complete = context.verified_answers_complete();
     let mut fragments = Vec::new();
     for answer in context.verified_answers() {
         if let Some(text) = render_verified_answer(answer) {