feat(github-app): slop gate mode + check surfacing

[JSONbored]

Parent roadmap: #525
Parent phase: #526

Background

Maintainers need to enforce a quality floor on incoming PRs. The CI already has GitHub contents:read, so it can score the real diff (deeper than the metadata-only miner side) and optionally augment with the existing opt-in LLM path.

Goal

Add a slopGateMode gate and surface slop in the existing check runs / PR panel.

Current Behavior

Gate modes exist (linkedIssueGateMode, duplicatePrGateMode, qualityGateMode) but there is no slop gate.

Desired Behavior

• Add slopGateMode (off / advisory / block, default advisory) + slopGateMaxRisk to RepositorySettings (src/types.ts), sibling to qualityGateMode.
• New buildSlopGateBlocker in src/rules/advisory.ts folded into evaluateGateCheck; computed in src/queue/processors.ts from the real PullRequestFileRecord[].
• Optional LLM augmentation via existing src/services/ai-summaries.ts: runs only on elevated/high deterministic bands, can only raise risk (never lower the deterministic floor), reuses AI_DAILY_NEURON_BUDGET.
• Surface via the existing Gittensory Gate / Gittensory Context checks + PR panel — no new comment.

Implementation Requirements

• New D1 migration for the two settings columns (mirror quality_gate_*). Depends on slop core (roadmap(pillar 1): anti-slop contribution quality layer #526 child).

Public/Private Output Boundaries

• All gate/panel text routed through sanitizeForCheckRun / the shared public-safe filter. No private scores or reward/trust language.

Acceptance Criteria

• slopGateMode off/advisory/block behaves correctly; block only fails when slopRisk > slopGateMaxRisk.
• LLM augmentation never weakens the deterministic gate; default advisory avoids false-positive blocks.

Testing Requirements

• npm run test:ci, 97%+ coverage. Gate-mode tests + sanitizer tests for the surfaced text + LLM-floor invariant test.

Sub-issues

• feat(github-app): deterministic slop gate (slopGateMode) #571 deterministic slop gate (slopGateMode)
• feat(github-app): opt-in LLM slop augmentation #572 opt-in LLM augmentation
• feat(github-app): render slop band in PR panel (visual) #573 render slop band in PR panel (visual — lower priority)