Add code quality infrastructure and comprehensive project documentation

[Copilot]

Establishes quality foundations and documentation standards for the SpdxModel library to improve maintainability, contributor experience, and code consistency.

Code Quality

• EditorConfig: C# style rules, .NET analyzer configurations, naming conventions
• Static Analysis: Microsoft.CodeAnalysis.NetAnalyzers for security, performance, and design checks
  • Suppressed CA1002 (List in APIs), CA1819 (array properties), CA1502 (complexity), CA1707 (test underscores) to align with existing patterns

Documentation

• README.md: Installation, quick start examples, API overview, feature list

  // Read SPDX document
  var json = File.ReadAllText("sbom.spdx.json");
  var document = Spdx2JsonDeserializer.Deserialize(json);
  
  // Create and serialize
  var doc = new SpdxDocument { /* ... */ };
  var output = Spdx2JsonSerializer.Serialize(doc);

• CONTRIBUTING.md: Development setup, coding standards, AAA test pattern, PR process
• CODE_OF_CONDUCT.md: Contributor Covenant v2.1
• SECURITY.md: Vulnerability reporting, security best practices
• QUALITY_IMPROVEMENTS_SUMMARY.md: Detailed implementation notes and prioritized future enhancements (mutation testing, integration tests, coverage reporting, benchmarks)

Automation

• Dependabot: Weekly NuGet and GitHub Actions updates with grouped dependencies
• Enhanced .gitignore: Coverage reports, IDE files, build artifacts

AGENTS.md Updates

Added sections on quality tools, coverage requirements (80%+ for new code), documentation standards (XML comments for public APIs), security guidelines, and CI/CD pipeline details.

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All changes are backward compatible. Build succeeds with 0 warnings, 153/153 tests pass across .NET 8/9/10.

Original prompt

what sort of quality improvements would be good to perform on this project. consider the code, tests, documentation, additional quality tools, and agents.md changes.

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