feat(github-app): first-time-contributor-aware gating

[JSONbored]

Parent roadmap: #525
Parent phase: #528

Background

Hard gates that false-positive on a newcomer's first PR scare off real contributors. Repeat low-quality contributors should still face the full gate.

Goal

First-time-contributor-aware gating: soften gates to advisory for newcomers, keep them hard for repeat offenders.

Current Behavior

GateCheckPolicy (advisory.ts:15) and gateCheckPolicy() (processors.ts:760) don't consider the PR author's history.

Desired Behavior

• Pass the author's RoleContext + outcome history into the gate; a per-repo firstTimeContributorGrace setting downgrades block→advisory for first-time/low-history authors while keeping block for repeat closed-unmerged authors.
• Reuses buildRoleContext (engine.ts:1404) + buildContributorOutcomeHistory (:1505).

Implementation Requirements

• New setting + migration; thread author context into evaluateGateCheck.

Public/Private Output Boundaries

• No contributor reward/trust internals in public output; grace is expressed as advisory vs block only.

Acceptance Criteria

• First-time author with a borderline PR gets advisory; repeat offender gets block.

Testing Requirements

• npm run test:ci, 97%+ coverage. Newcomer vs repeat-offender fixtures.