feat(github-app): focus-manifest policy as gate input

[JSONbored]

Parent roadmap: #525
Parent phase: #528

Background

Repo owners can declare policy in a focus manifest, but today it only produces advisory guidance — CI doesn't enforce it.

Goal

Wire focus-manifest policy into the gate so declared blocked paths / required-linked-issue / test expectations become configurable gate blockers.

Current Behavior

FocusManifest (src/signals/focus-manifest.ts:13) feeds FocusManifestGuidance (advisory only); evaluateGateCheck ignores it.

Desired Behavior

• A manifestPolicyGateMode setting makes blockedPaths, linkedIssuePolicy:"required", and testExpectations enforceable gate blockers, surfaced through the existing Gate check.

Implementation Requirements

• Extend GateCheckPolicy + evaluateGateCheck to consume manifest policy; new setting + migration. Reuse the existing manifest parser/guidance.

Public/Private Output Boundaries

• Gate text through sanitizeForCheckRun; manifest content is repo-public already.

Acceptance Criteria

• A PR touching a manifest blockedPath or missing a required test fails the gate when enabled.

Testing Requirements

• npm run test:ci, 97%+ coverage. Manifest-policy gate fixtures.