fix(compaction): admit prompts during breaker cooldown

[code-yeongyu]

Summary

• let prompt, final-payload, scheduled-continuation, and retry admission proceed when opportunistic compaction is unavailable because the circuit breaker is cooling down
• keep genuine oversized payloads, overflow compaction, and non-breaker cancellation fail-closed
• add deterministic regression coverage and a hermetic real-CLI outage/continuation scenario

This supersedes #532 on current main, preserving the original contributor's implementation and co-author attribution. It does not overlap #527's separate blocking-route breaker/cap enforcement in the builtin compaction extension.

RED -> GREEN

• RED on origin/main: 531-compaction-cooldown-admission.test.ts failed with RequiredCompactionError at _enforceCompactionBeforeProvider; non-breaker control passed.
• GREEN after fix: 2/2 issue regression tests pass.

Verification

• Scoped compaction/session tests: 45 files, 360 tests passed.
• npm run check: passed.
• Senpi CLI QA: local-ignore/qa-evidence/20260731-issue-531-breaker-admission/cli-qa.log
  • 4/4 checks passed
  • post-trip compaction rejections: 0/7
  • provider requests continued: 21
  • compaction lifecycle pairs: 29 starts / 29 ends
  • real auth unchanged

Closes #531

---

Summary by cubic

Admit prompts during compaction breaker cooldown to prevent bricked sessions during provider outages. Non-breaker cancellations and true overflow still block as before.

• Bug Fixes
  • Allow prompt, final-payload, scheduled continuation, and retry admission to proceed when compaction was cancelled due to circuit-breaker cooldown.
  • Preserve fail-closed behavior for oversized payloads, overflow-driven compaction, and non-breaker cancellations.
  • Add a regression test and a hermetic CLI outage scenario to validate no spinner leaks and that turns keep reaching the provider.

^{Written for commit 48682cb. Summary will update on new commits.}