fix(context): persist fallback compaction breaker

[kshitijk4poor]

Summary

Fallback-only compaction loops now trip a durable, boundary-scoped circuit breaker across normal turns, session rotation, gateway recreation, and usage-less responses.

The original PR correctly identified deterministic fallback compactions as degraded, but its shared usage counter was reset by ordinary fitting responses between compaction episodes. This salvage preserves @LeonSGP43's commit and adds the lifecycle-complete breaker as a separate maintainer follow-up.

Changes

• count fallback quality when a completed compaction boundary commits, independently of provider usage
• persist the fallback streak per session and carry it across rotation/in-place/fresh gateway agents
• keep ordinary fitting responses from erasing fallback history; reset only on a healthy compaction, a real runtime switch, or a new session
• prevent stale Hermes fallback state from tainting Codex-native compaction
• gate every automatic built-in compression entrypoint while preserving forced /compress
• add production-seam tests for usage-less turns, context regrowth, rotation, persistence, Codex, model switches, and schema reconciliation

Validation

Check	Result
Canonical targeted suite	638 passed
Final focused suite	400 passed
Real SessionDB E2E	fallback 1 → 1 → 2/block → reload/block → healthy/reset
Mutation	disabling the fallback increment fails 2 regression tests
Ruff / ty diff	no new diagnostics (excluding symmetric ty panic noise)
git diff --check	clean

Closes #63008.

Credit: salvages #63018 by @LeonSGP43; contributor commit and authorship preserved.