Reviewer cohort reuse loops on files no agent can cover

[rianjs]

Reviewer cohort reuse should tolerate inherently unassigned files

Problem

After a fresh review of yakshed PR 7, .github/workflows/ci.yml is intentionally reported by the normal coverage gate as unassigned because no current reviewer agent glob matches it. The saved cohort then makes every plain follow-up fail before reviewer execution with:

saved reviewer cohort leaves changed file ".github/workflows/ci.yml" uncovered; pass --fresh-session

Running --fresh-session selects the same catalog and cannot make the file coverable, so the guidance creates a loop and discards useful reviewer-session continuity.

Root cause

First-run selection permits files that match no selected agent to remain in the existing incomplete_unassigned coverage bucket. Cohort rebasing applies a stricter invariant: every changed file must be assigned to a saved member, even when no agent in the current catalog can match it. That makes later-run behavior inconsistent with first-run behavior.

Change

Keep the fixed-cohort contract and its useful incompatibility failures, but distinguish recoverable from inherent gaps while rebasing:

• Reassign new files to saved broad members or saved scoped members whose globs match, as today.
• If no saved member can take a file but another current catalog agent can match it, retain the terminal --fresh-session guidance because reselection can help.
• If no current catalog agent can match the file, leave it unassigned and continue with the saved cohort. The existing reviewer-coverage gate remains authoritative and must report incomplete_unassigned; this does not silently make approval eligible.
• Preserve saved reviewer ordering and provider-session reuse.
• Preserve failures for adapter/runtime incompatibility, missing cohort members, and undersized --max-agents.
• Update the lifecycle documentation to describe the distinction.

TDD verification

1. Add a failing rebase test with a saved scoped reviewer covering main.go and an unmatched .github/workflows/ci.yml; expect successful cohort reuse with only main.go assigned.
2. Prove the downstream coverage summary still contains an unassigned entry for the workflow file.
3. Add/retain a failing case where a catalog agent outside the saved cohort can match the newly uncovered file; expect --fresh-session guidance.
4. Run focused pipeline tests, race-sensitive tests if the changed path has concurrent state, and make check.

Empirical definition of done

Build the combined candidate from draft PR #561. Using the established yakshed PR 6 and PR 7 cohorts, run normal non-posting follow-up reviews without --fresh-session:

• PR 6 completes without dossier process-vocabulary rejection.
• PR 7 completes without the saved-cohort uncovered-file terminal error, preserves reviewer reuse, and reports .github/workflows/ci.yml through the ordinary unassigned coverage entry.

Both commands must exit successfully. No yakshed review is posted.

Delivery

Implement as a separate commit after 6c59cfb on draft PR #561. Update that PR to close this issue as well as #559. Run one independent TDD coverage assessment for the combined PR, the normal cr review loop, CI, and squash-merge only after both empirical gates pass.