feat(github-app): gate appeal/override command + configurable settings

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Parent roadmap: #525
Parent phase: #528

Background

Gates can false-positive. Maintainers need a relief valve, and the new surfaces need to be tunable and previewable before they are enabled. The appeal/override must ship in the same wave as any gate expansion.

Goal

Add a @gittensory gate-override command plus new configurable settings wired through the existing dry-run preview.

Current Behavior

A @gittensory command catalog and a settings-preview dry-run exist; there is no override command and no settings for the new surfaces.

Desired Behavior

• New @gittensory gate-override command (catalog in src/github/commands.ts), authorized via a new gateOverridePolicy, scoped to the current headSha (a new push re-evaluates — no permanent bypass), audited.
• New settings: checkRunAnnotations, issueTriageMode, reviewerRoutingMode, slopGateMode, badgeEnabled — all surfaced in the existing settings-preview so maintainers see the effect before enabling.

Implementation Requirements

• Reuse isAuthorizedCommandActor; re-run createOrUpdateGateCheckRun with a neutral conclusion on override; record an audit event.

Public/Private Output Boundaries

• Override comment stores actor login + reason only; routed through the public-safe filter.

Acceptance Criteria

• An authorized override flips the gate for the current headSha only; a new push re-evaluates. Settings preview reflects all new toggles.

Testing Requirements

• npm run test:ci, 97%+ coverage. Authorization tests + headSha-scoping test (override clears on new push).