fix(review): hold PRs whose review cannot complete, never "safe to merge"

[JSONbored]

What

When preflight cannot complete the review (the repo's review lane is unknown/inactive, so the PR isn't eligible for a confident automated recommendation), the unified disposition previously still rendered an otherwise-ready PR as Approve/Merge. The preflight hold only fed the advisory readiness score, never the merge/close disposition — so an incomplete review could read as "safe to merge."

This threads preflightHeld (preflight.status === "hold") through the comment bridge into deriveUnifiedStatus, which downgrades an otherwise-ready status to a manual-review hold.

Why it's safe

• The downgrade is applied last and only to a would-be-ready status, so it can only ever downgrade, never approve — a real CI / merge-state / gate block still wins.
• A gate merge verdict carrying advisory blockers stays authoritative-ready by design — the gate already weighed those. Tightening that behavior belongs in the gate's confidence/approval bars, not this renderer. (Called out inline; see fix(review): disposition recommends approve/merge while blockers are active (+ approval-bar / confidence level may be too loose) #2002 for the separate discussion.)

Also corrects the preflight-hold validation-component message to describe the real cause (review lane unavailable) instead of a nonexistent "blocker."

Tests

• unified-comment.test.ts — a preflight hold downgrades a gate merge verdict to held; a clean merge with no hold still renders ready (downgrade-only invariant); a gate merge with advisory blockers stays authoritative-ready.
• unified-comment-bridge.test.ts — a passing PR with preflightHeld renders > [!WARNING] / Manual Review, never > [!TIP] / Approve-Merge.
• signals-coverage.test.ts — updated to the corrected preflight-hold message.

Full unsharded test:coverage green; typecheck green.

Advances #1936. Refs #2002.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.69%. Comparing base (5e82d8e) to head (af0c162).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2008   +/-   ##
=======================================
  Coverage   95.69%   95.69%           
=======================================
  Files         222      222           
  Lines       24584    24586    +2     
  Branches     8923     8925    +2     
=======================================
+ Hits        23526    23528    +2     
  Misses        433      433           
  Partials      625      625           

Files with missing lines	Coverage Δ
src/queue/processors.ts	90.02% <ø> (ø)
src/review/unified-comment-bridge.ts	96.15% <100.00%> (+0.03%)	⬆️
src/review/unified-comment.ts	99.44% <100.00%> (+<0.01%)	⬆️
src/signals/engine.ts	97.36% <100.00%> (ø)

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