feat(config): add review.auto_merge_summary read-only knobs surface

[JSONbored]

Add review.auto_merge_summary (bool, default off) — renders a READ-ONLY summary in the unified comment of which auto-merge conditions currently pass/fail (CI green, gate passing, mergeable-clean, valid linked issue). Surface only; does NOT change the merge/close DECISION. Deterministic; off ⇒ byte-identical. Decomposed from #1959.

Deliverables

• Add autoMergeSummary: boolean | null to FocusManifestReviewConfig in src/signals/focus-manifest.ts via normalizeOptionalBoolean
• Render a collapsible read-only conditions table in src/review/unified-comment.ts from ALREADY-computed readiness signals (do not re-derive or call any decision path); gated by the knob, default off
• Wire parse + reviewConfigToJson + present + EMPTY literals
• Unit tests: on renders the table from injected signals, off byte-identical, invalid warns; branch coverage on both arms — and an explicit assertion that no merge/close decision code is invoked
• Document review.auto_merge_summary in .gittensory.yml.example, clearly marked read-only

References

• src/review/unified-comment.ts:12 (readiness signals in UnifiedCommentContext)
• src/signals/focus-manifest.ts:421 (normalizeOptionalBoolean)
• .gittensory.yml.example
• part of #1959 (epic #1953)

Part of #1959.

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size: S · gittensor:feature

[linear-code]

JSO-2168