test(core): add Vitest setup and unit tests

[runtimebug]

Summary

Implements Task 2 of Phase 1 from the Evolution Plan: Vitest setup + unit tests for parseUnifiedDiff, config loading, and response parsing.

Changes

• Install Vitest and @vitest/coverage-v8 for testing
• Create vitest.config.ts with coverage settings
• Add 15 tests for parseUnifiedDiff.ts
• Add 26 tests for config/index.ts
• Add 20 tests for OpenAI response parsing
• Add test fixtures for diffs, configs, and responses
• Add GitHub Actions CI workflow for automated testing
• Update package.json with test scripts
• Update .gitignore to exclude coverage/

Test Coverage

• parseUnifiedDiff: 15 tests covering basic functionality, line number annotation, multiple hunks, context lines, and edge cases
• config loading: 26 tests covering config file discovery, CLI merging, validation, and error handling
• response parsing: 20 tests covering summary/inline styles, various separators, file paths, and edge cases

Total: 61 unit tests

How to Test

npm install
npm test          # Run tests once
npm run test:watch    # Run tests in watch mode
npm run test:coverage # Run tests with coverage

Checklist

• Code compiles: npm run build
• All tests pass: npm test
• PR title follows Angular commit convention
• No secrets or API keys included