feat(review): wire review.auto_merge_summary to the unified comment, or remove the dead render primitive

[JSONbored]

Context

Found while auditing review-control wiring for #1681. .gittensory.yml's review.auto_merge_summary is
parsed, and a pure render primitive for it already exists and is unit-tested — but neither the field nor
the primitive is ever wired into the actual comment pipeline. Setting it today has no visible effect.

Evidence

• buildAutoMergeSummaryCollapsible (src/review/unified-comment.ts:229, feat(config): add review.auto_merge_summary read-only knobs surface #2051) builds a real
  UnifiedCollapsible from AutoMergeSummarySignals, with its own dedicated pure-function test suite
  (test/unit/review-auto-merge-summary.test.ts).
• grep -rn "buildAutoMergeSummaryCollapsible" across the repo shows exactly one caller: its own test
  file. It is never imported or called from src/review/unified-comment-bridge.ts or
  src/queue/processors.ts — the two places that assemble and post the real unified comment.
• grep -rn "autoMergeSummary" src/ (the manifest field itself) returns nothing outside the parser/type
  definitions — no resolver reads manifest.review.autoMergeSummary, unlike every other opt-in render
  toggle (review.changed_files_summary, review.effort_score, etc.), which all flow through
  resolveReviewPromptOverrides into a real src/queue/processors.ts call site.

The ask

Pick one:

1. Wire it: add autoMergeSummary to resolveReviewPromptOverrides
   (src/signals/focus-manifest.ts), thread it through src/queue/processors.ts the same way
   changedFilesSummaryEnabledForReview/effortScoreEnabledForReview are threaded, and call
   buildAutoMergeSummaryCollapsible from the unified-comment assembly when a repo opts in — plus e2e
   test coverage mirroring the changed_files_summary/effort_score tests in test/unit/queue.test.ts.
2. Deprecate/remove it: if there's no product plan to surface this collapsible, drop the manifest
   field, its parsing, and buildAutoMergeSummaryCollapsible (or leave the primitive but remove the
   config-facing field/docs so it isn't advertised as configurable).

Not presuming which — flagging so a maintainer can decide.

Related

• Found during feat(review): test review controls end to end #1681 (review-controls end-to-end test audit).

[linear-code]

JSO-3168