Apply all coding agent improvements to NuGet Cache Tool

[Copilot]

Ran all six coding agents against the repository to identify and apply improvements across code correctness, test coverage, documentation, and requirements traceability.

Changes

Correctness — stderr vs stdout

• Context.WriteError() and Program.Main catch blocks now write to Console.Error instead of Console.Out — error messages no longer pollute stdout when consumers pipe package paths
• Updated affected unit tests to redirect Console.Error accordingly

Reliability — log file flushing

• Context.OpenLogFile opens StreamWriter with AutoFlush = true so log entries survive unexpected process termination

Test Coverage

• Context_WriteError_Silent_DoesNotWriteToConsole — covers silent-mode suppression of stderr (previously only stdout path was tested)
• Context_WriteError_WritesToLogFile — covers error message persistence to log file

Requirements Traceability (requirements.yaml)

• Added missing test linkages: Program_Version_ReturnsNonEmptyString → NC-CLI-002; IntegrationTest_ValidateWithResults_GeneratesJUnitFile → NC-CLI-006
• NC-CLI-009 — error reporting and non-zero exit codes (backed by existing tests)
• NC-CLI-010 — invalid argument rejection (backed by existing tests)
• NC-SEC-001 (new Security section) — path traversal prevention via SafePathCombine (backed by all 7 PathHelpers tests)

Documentation

• docs/guide/guide.md: documents stderr for error output; clarifies --results extension determines format (.trx / .xml)

Consistency

• Test project LangVersion updated from 12 → latest to match the main project
• AGENTS.md Tech Stack updated from C# 12 → C# (latest) to match

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code quality improvement

Related Issues

Pre-Submission Checklist

Build and Test

• Code builds successfully: dotnet build --configuration Release
• All unit tests pass: dotnet test --configuration Release
• Self-validation tests pass:
  dotnet run --project src/DemaConsulting.NuGet.CacheTool --configuration Release --framework net10.0
--no-build -- --validate
• 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

Quality Checks

• Spell checker passes: cspell "**/*.{md,cs}"
• Markdown linter passes: markdownlint "**/*.md"
• 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

All 43 tests pass across .NET 8, 9, and 10. Code review returned no comments.

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