feat: AI-powered smart import detection for multi-language PRs

[oddur]

Summary

• Adds a "Smart imports" toggle that uses Claude Haiku to detect local file imports across all languages (C#, Rust, Python, Go, etc.), replacing the regex-only ES6 parser when enabled
• Existing regex behavior is preserved as the default — new path is opt-in via toggle
• Shows neighbor file count on the overview slide when files are found

How it works

• callClaudeQuick() in lib/agent.ts — lightweight, non-streaming CLI helper
• extractImportsWithClaude() in lib/github.ts — sends changed file contents to Haiku with a system prompt that returns a JSON array of repo-relative import paths
• getNeighborFiles accepts a useSmartImports flag; when false, existing regex logic runs unchanged

Test plan

• Toggle OFF: generate review for JS/TS PR — confirm same behavior as before
• Toggle ON: generate review for C# PR — confirm neighbor files contain .cs files
• Toggle ON: generate review for JS/TS PR — confirm it still works
• Verify overview slide shows "{N} additional files included for context"
• Verify logs show Claude's detected imports and fetched neighbor files