Summary

This PR adds automated detection of Git merge conflict markers in pull requests to prevent accidentally merging unresolved conflicts into the PowerShell repository.

What This PR Does

1. Adds Merge Conflict Marker Detection

   • New composite action at .github/actions/infrastructure/merge-conflict-checker
   • Integrated into linux-ci.yml workflow as a new merge_conflict_check job
   • Only runs on pull requests in authorized repositories (azure/* or PowerShell)
   • Blocks merging if any conflict markers are found (added to ready_to_merge dependencies)

2. Implements Detection Logic in PowerShell Module

   • Added Test-MergeConflictMarker function in tools/ci.psm1
   • Uses PowerShell's Select-String cmdlet for efficient pattern matching with automatic line numbering
   • Detects all three Git conflict marker types: <<<<<<<, =======, >>>>>>>
   • Provides detailed GitHub Actions job summary with:
     • Total files checked and conflicts found
     • Relative file paths (repo-relative, not full agent paths)
     • Precise line numbers for each conflict marker
     • Formatted markdown tables for easy reading

3. Adds Developer Documentation

   • .github/instructions/powershell-module-organization.instructions.md - Guidelines for organizing PowerShell code across ci.psm1, build.psm1, and packaging.psm1 modules
   • .github/instructions/powershell-automatic-variables.instructions.md - Comprehensive guide on avoiding PowerShell automatic variables like $matches, $_, $args, and $input
   • Action-specific README documentation

Technical Implementation

Detection Logic:

• Regex pattern: ^(<{7}|={7}|>{7})(\s|$) - matches 7 consecutive markers at line start, followed by whitespace or end-of-line
• Properly matches real Git conflict markers with trailing content (e.g., <<<<<<< HEAD)
• Skips binary files, deleted files, and directories
• Uses relative paths in output for better readability

Code Quality:

• Follows PowerShell best practices (capital Function keyword, singular parameter names)
• Avoids PowerShell automatic variable conflicts (uses $matchedLines instead of $matches)
• Minimal YAML - complex logic moved to reusable module function
• Secure file handling using environment variables

Security:

• Repository owner check prevents execution in unauthorized forks
• Only processes changed files (retrieved via GitHub API)
• Proper error handling with ErrorAction Stop

Files Changed

New Files:

• .github/workflows/linux-ci.yml - Added merge_conflict_check job
• .github/actions/infrastructure/merge-conflict-checker/action.yml - Composite action
• .github/actions/infrastructure/merge-conflict-checker/README.md - Action documentation
• .github/instructions/powershell-module-organization.instructions.md - Module organization guidelines
• .github/instructions/powershell-automatic-variables.instructions.md - Automatic variables guide

Modified Files:

• tools/ci.psm1 - Added Test-MergeConflictMarker function

Next Steps (Not Included in This PR)

1. Unit Tests: Add Pester tests for Test-MergeConflictMarker function in test/infrastructure/ (per reviewer request)
2. Integration Testing: Verify the workflow works correctly in live PR scenarios

Benefits

1. Prevents Merge Conflicts: Automatically detects and blocks PRs with unresolved conflict markers
2. Clear Feedback: Provides developers with exact file paths and line numbers to fix
3. Maintainable Code: Well-organized PowerShell in modules with comprehensive documentation
4. Future-Proof: Instructions help prevent common PowerShell pitfalls for future contributors

Fixes #26349

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