Modernize repository configuration following template consistency review

[Copilot]

Pull Request

Description

Aligns SonarMark repository with TemplateDotNetTool patterns based on repo-consistency-agent findings. Addresses configuration drift in markdownlint format, gitignore coverage, and solution file format.

Solution Format Migration

• Migrated from legacy .sln to XML-based .slnx format using dotnet solution migrate
• Updated .vscode/tasks.json references accordingly
• .slnx provides better diff-ability and merge conflict resolution

Markdownlint Configuration Modernization

• Replaced .markdownlint.json with .markdownlint-cli2.jsonc
• Added built-in ignore patterns for node_modules and AGENT_REPORT_*.md
• Removed redundant CLI arguments from lint scripts (now handled by config)

.gitignore Enhancement

• Added AGENT_REPORT_*.md pattern to prevent committing agent communication files
• Added comprehensive IDE patterns (VS, VS Code, Rider)
• Added additional temporary file patterns (*.suo, *.sln.iml, etc.)

Documentation Updates

• Added agent markdown exception to CONTRIBUTING.md for .github/agents/*.md files
• Updated lint command examples to reflect current usage
• Fixed yamllint invocation (removed incorrect npx prefix - yamllint is Python-based)

Minor Fixes

• Added "buildnotes" to .cspell.json dictionary

Type of Change

• Code quality improvement
• Documentation update

Related Issues

N/A - Proactive consistency maintenance

Pre-Submission Checklist

Build and Test

• Code builds successfully: dotnet build --configuration Release
• All tests pass: dotnet test --configuration Release
• Code produces zero warnings

Code Quality

• Code formatting is correct: dotnet format --verify-no-changes
• New code has appropriate XML documentation comments
• Static analyzer warnings have been addressed

Linting

• Markdown linter passes: npx markdownlint-cli2
• Spell checker passes: npx cspell "**/*.{md,cs}" --no-progress
• YAML linter passes: yamllint .

Testing

• Added unit tests for new functionality
• Updated existing tests if behavior changed
• All tests follow the AAA (Arrange, Act, Assert) pattern
• Test coverage is maintained or improved

Documentation

• Updated README.md (if applicable)
• Updated ARCHITECTURE.md (if applicable)
• Added code examples for new features (if applicable)
• Updated requirements.yaml (if applicable)

Additional Notes

No functional code changes. All 76 tests pass across .NET 8.0, 9.0, and 10.0. Code review and CodeQL checks passed.

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