Phase 1 low-risk backend batch (on top of Batch A). Both byte-verified vs PR head,
Codex SAFE, 64 own + 749 regression tests pass, coexists with Batch A's #5833.
- #5828 (ai-ag2026): frame CORS OPTIONS preflight with Content-Length: 0 (keep-alive hang fix)
- #5834 (ai-ag2026): parse session _index.json from bytes not decoded str (all 10 sites)
Co-authored-by: t <a@b>
Co-authored-by: ai-ag2026 <ai-ag2026@users.noreply.github.com>
