Improve multi-turn guardian transcript review

[maja-openai]

Summary

• change guardian follow-up reviews to send transcript deltas instead of replaying the full transcript
• keep transcript numbering consistent across reused and forked guardian review sessions
• limit delta tool evidence to the latest turn and add regression coverage for follow-up prompt behavior

Testing

• just fmt
• cargo test -p codex-core build_guardian_transcript_keeps_original_numbering
• cargo test -p codex-core build_guardian_follow_up_prompt_uses_transcript_delta_and_only_latest_turn_commands
• cargo test -p codex-core guardian_reuses_prompt_cache_key_and_appends_prior_reviews
• cargo test -p codex-core guardian_parallel_reviews_fork_from_last_committed_trunk_history
• just fix -p codex-core