feat(config): expose remaining DB-only gate settings in .gittensory.yml (config-as-code parity)

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Follow-up to #551 (merge-readiness aggregate gate). The DB-settable mergeReadinessGateMode shipped, but the config-as-code parity was deferred: .gittensory.yml can't yet set the composite via gate.mergeReadiness.

Why deferred

The other gate modes map from the manifest in src/signals/focus-manifest.ts (e.g. if (gate.slopMode !== null) effective.slopGateMode = gate.slopMode;). Adding gate.mergeReadiness needs the manifest gate-schema parser extended to recognize + normalize the new key — more than the one-line mapping, so it was split out to keep #551 focused.

Deliverables

• Add mergeReadiness: off|advisory|block to the .gittensory.yml gate schema + parser (focus-manifest.ts)
• Map it onto effective.mergeReadinessGateMode in resolveEffectiveSettings (manifest wins over DB, mirroring the other gate modes)
• Test: gate.mergeReadiness: block in yml drives the composite

Context

src/signals/focus-manifest.ts (~488 the gate-mode mapping, ~479 resolveEffectiveSettings). Relates #551, #555 (focus-manifest-as-gate).

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Scope broadened (after #552 merged)

#552 shipped firstTimeContributorGrace as a DB-only setting and introduced the same config-as-code parity gap as #551's mergeReadinessGateMode: src/signals/focus-manifest.ts maps every other gate field from the manifest (e.g. gate.slop.mode → slopGateMode), but neither mergeReadinessGateMode nor firstTimeContributorGrace is mappable from .gittensory.yml yet.

This is a recurring debt: each new DB gate setting ships without its manifest mapping. Broadening this issue to close both in one focused change so the config-as-code gate surface stays complete (a prerequisite for the agent layer reading/proposing full gate policy as code).

Deliverables (updated)

• gate.mergeReadiness: off|advisory|block → effective.mergeReadinessGateMode (manifest wins over DB)
• gate.firstTimeContributorGrace: true|false → effective.firstTimeContributorGrace
• Extend the manifest gate-schema parser + gate.present predicate + resolveEffectiveSettings mapping for both
• Tests: each new key in yml drives the effective setting end-to-end through the gate

Relates #551, #552, #555 (focus-manifest-as-gate).