feat: Auto-detect CI environment for output format

[Kilo59]

Description

Add auto-detection of CI environments to ruff-sync so that it defaults to the appropriate output format without requiring manual flags.

Proposed Mapping

• GITHUB_ACTIONS=true -> --output-format github
• GITLAB_CI=true -> --output-format gitlab

Environment Reference

GitHub Actions

GitHub naturally sets the following variables in a runner:

• GITHUB_ACTIONS: Always set to true when GitHub Actions is running the workflow.
• GitHub Documentation: Default environment variables

GitLab CI/CD

GitLab sets the following variables in a runner:

• GITLAB_CI: Always set to true for all jobs executed by GitLab CI/CD.
• GitLab Documentation: Predefined variables

Technical Details

• Update Arguments resolution in src/ruff_sync/cli.py to check for these environment variables when output_format is not explicitly provided.
• Ensure that an explicit --output-format flag still takes precedence over auto-detection.