M3: Add local AI provider interface and Claude adapter

[dbrosio3]

Summary

• add a provider-backed local AI review boundary under src/ai with shared review payload assembly and output parsing
• implement the first Claude adapter and wire it into pushgate pre-push with blocking vs advisory failure handling
• extend unit, runner, and installed-hook coverage for AI payloads, findings, provider failures, and skip behavior

Why

• issue M3: Add local AI provider interface and Claude adapter #10 requires Pushgate local AI to flow through a provider interface instead of a Claude-specific core path
• this keeps the current Claude workflow available while creating the seam for later issues around guardrails, structured findings, and additional providers

Validation

• pnpm test

Closes #10