fix: make transcript rewrite atomic to prevent data loss on crash

[tomqiaozc]

Summary

• Add SessionDB.rewrite_messages() that runs DELETE + all INSERTs inside a single _execute_write() transaction — a crash during rewrite rolls back to the original messages instead of leaving an empty or partial transcript
• Use tempfile + fsync + os.replace for JSONL rewrites — a crash mid-write preserves the original file intact
• Replaces the previous non-atomic clear_messages() + N × append_message() sequence in rewrite_transcript()

Test plan

• test_sqlite_rewrite_is_atomic — verifies rewrite_messages works end-to-end
• test_jsonl_rewrite_preserves_original_on_error — verifies original JSONL file survives when os.replace fails
• test_reasoning_survives_rewrite — existing regression test still passes
• All 62 session tests pass, all 9 dedup tests pass

Closes #8029

🤖 Generated with Claude Code

[alt-glitch]

Likely duplicate of #14774 — same root cause (non-atomic transcript rewrite in /retry, /undo, /compress causing data loss on crash). #14774 is the active fix for #8029.

[teknium1]

Thanks for the careful durability work. This is already implemented on current main — automated hermes-sweeper review.

• gateway/session.py:2307-2327 delegates transcript replacement to the canonical database implementation and returns failure to callers when that write fails.
• hermes_state.py:3632-3678 performs the delete, reinsert, and metadata updates inside one _execute_write transaction; its rollback behavior protects the original transcript if an insert fails.
• tests/gateway/test_session.py:1536-1576 explicitly forces a mid-rewrite failure and verifies the preexisting messages survive.
• The JSONL fallback addressed by this PR no longer participates in transcript rewrites: commit 351fdcc6e removed it, and gateway/session.py:2329-2342 now uses state.db only.

The transactional implementation shipped in 2f9243c333 and is contained in release v2026.5.28. The duplicate relationship noted by @alt-glitch with #14774 is consistent with the current implementation.