Store guardian transcript boundary on review session

[charley-oai]

Summary

• store the parent transcript checkpoint on the cached guardian review session instead of rollout reconstruction or parent session state
• slice guardian transcript evidence from that checkpoint so follow-up reviews only include transcript since the last terminal guardian review
• preserve the checkpoint when the cached guardian trunk is recreated for a config change and add coverage for approved and denied follow-up reviews

[charley-oai]

@codex review

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Reviewed commit: 8d9ff1943f

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[charley-oai]

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Codex Review: Didn't find any major issues. 🎉

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[charley-oai]

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Codex Review: Didn't find any major issues. 🎉

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[charley-oai]

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Reviewed commit: be7bc5c9ae

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[Hardanish-Singh]

LGTM

[Raymondriter]

I'd close this as superseded rather than request a rebase.

The follow-up transcript slice this PR wanted is already on main as GuardianTranscriptCursor + GuardianPromptMode::{Full, Delta}, stored on the review session (last_reviewed_transcript_cursor) and copied through fork snapshots. Main also version-checks against parent history (so compaction/rewrite falls back to a full prompt), keeps root authorization out of the delta, and after #41215 resets the cursor when reviewer compaction drops the original transcript.

This branch is from March (be7bc5c, last code Mar 24) and is mergeable_state: dirty against current main. The design here is a raw raw_items().len() index on the session manager, which is weaker than what landed and has its own holes even on that snapshot:

• Sequential same-turn reviews (two tools, no history append) slice [len..] and send <no retained transcript entries>. Tests only cover follow-up after a new user turn.
• Abort / timeout / fail-closed still call mark_terminal_guardian_review_boundary, so a review that never assessed the current history still hides it from the next one.
• Only replace_history clears the checkpoint; replace_compacted_history is untouched.
• Config-change trunk recreate keeps the parent slice while spawning with initial_history: None, so the new reviewer loses both prior assessments and original user authorization. Main starts a new trunk in Full mode instead.

CI was green on Mar 24 against then-main; that does not apply now. No maintainer review (the Aug 1 LGTM is unaffiliated). Please reopen from current main only if some delta-cursor case is still missing. I don't think this patch is the vehicle.