Pluggable AI backend (provider seam: free / BYOK / local)

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Part of #975.

Context

The review pipeline currently calls a single hosted AI provider. Self-hosters need to choose an AI backend without touching the pipeline or the decision policy.

Deliverables

• A single AI-provider interface — one call (system + input → structured result) used everywhere the pipeline invokes a model.
• Refactor the existing model call sites onto this interface (no change to the consensus / decision logic above it).
• Built-in providers: hosted/free, BYOK API (OpenAI/Anthropic-compatible), local (Ollama / OpenAI-compatible endpoint).
• Provider selection via config + a fallback chain; fail-safe to manual review on any provider failure (never a wrong automated action).
• Per-project model selection.

Acceptance criteria

• Switching providers is a config change only; the pipeline + decision policy are untouched.
• A provider outage/timeout/parse-failure degrades down the chain and ultimately to manual — never an incorrect merge/close.

Notes

• Subscription providers (Claude Code / Codex) plug into this same seam — tracked separately.

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Closing as completed (2026-07-01 issue audit): Pluggable AI provider seam shipped (src/selfhost/ai.ts + ai-config.ts: free/BYOK/local).