feat(github-app): opt-in LLM slop augmentation

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Parent roadmap: #525
Parent epic: #532

Goal

Opt-in LLM augmentation: runs only on elevated/high deterministic bands, can only RAISE risk (never lower the deterministic floor), reuses the AI budget.

Reuse

rewriteSignalBundleWithAi (src/services/ai-summaries.ts), AI_DAILY_NEURON_BUDGET.

Acceptance

LLM never weakens the deterministic gate; budget + fallback respected.

Testing

npm run test:ci, 97%+ coverage. Public/private output boundary preserved; deterministic where applicable.

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Verified shipped via #724/#728/#737 (2026-06-17 triage). src/services/ai-slop.ts runGittensoryAiSlopAdvisory — gated on confirmed contributor, AI daily budget, fail-safe fallback; advisory-only (code ai_slop_advisory, never a blocker, never lowers the deterministic floor — processors.ts:1198); BYOK via #728. Closing.