docs: add AGENTS.md file with guidelines for AI agents

AGENTS.md

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+# Agent Guidelines for @semantic-release/gitlab
+
+## Commands
+
+- **Test**: `npm test` or `npm run test`
+- **Lint**: `npm run lint`
+- **Fix linting**: `npm run lint:fix`
+- **Coverage**: Tests include coverage reporting via c8
+
+## Code Style
+
+- **Node.js Version**: 20.8.1+
+- **Module type**: ES modules (`"type": "module"`)
+- **Formatting**: Prettier with 120 character line width and ES5 trailing commas
+- **Testing**: AVA test framework
+- **Coverage**: c8 for code coverage
+
+## Development Guidelines
+
+- **Perform minimal changes**: Focus on the specific issue and make the smallest possible changes
+- **Focus on reviewers**: Write clear, understandable code that is easy to review
+- **Use semantic commits**: Follow conventional commit format (e.g., `feat:`, `fix:`, `docs:`, `test:`)
+- **Run tests and formatting before committing**: Always run `npm test` and `npm run lint` before making commits
+
+## Project Structure
+
+- `lib/` - Core library modules
+- `test/` - Test files (using AVA)
+- `index.js` - Main entry point
+- Configuration files support semantic-release conventions
+
+## Testing Requirements
+
+- All changes must maintain or improve test coverage
+- Tests use nock for HTTP mocking
+- Integration tests verify GitLab API interactions
+- Run `npm test` to execute the full test suite
+
+## Semantic Release Integration
+
+This is a semantic-release plugin, so changes should follow semantic versioning principles and conventional commit formats for automated releases.