[plan] Implement parallel multi-file actionlint execution #7649

Objective

Enable actionlint to process multiple workflow files in a single Docker run for 2-3x faster validation.

Context

Currently, actionlint runs sequentially with one Docker container per file. The actionlint CLI supports multiple files as arguments, allowing batch processing.

Current flow: docker run ... file1.yml → docker run ... file2.yml → docker run ... file3.yml

Improved flow: docker run ... file1.yml file2.yml file3.yml

Approach

1. Modify runActionlintOnFile() in pkg/cli/actionlint.go to accept multiple files
2. Update the calling code to batch all workflow files into a single execution
3. Ensure JSON output parsing handles multi-file results correctly
4. Update error display to show which file each error came from
5. Maintain timeout protection (may need to adjust based on file count)

Files to Modify

• Update: pkg/cli/actionlint.go - Modify Docker execution logic
• Update: pkg/cli/compile_command.go - Batch files before calling actionlint
• Update: pkg/cli/actionlint_test.go - Add multi-file test cases

Acceptance Criteria

• Multiple workflow files processed in single Docker run
• JSON output correctly parsed for all files
• Error messages clearly indicate source file
• Performance improvement measurable (test with 3+ workflows)
• Timeout adjusted appropriately for multiple files
• Existing single-file functionality preserved

Performance Impact

Expected: 2-3x faster for projects with multiple workflows (eliminates Docker startup overhead per file)
Related to #7648

AI generated by Plan Command for discussion #7472