docs: add open source community health files and enable XML docs

[akunzai]

Changes

• Add .github/SECURITY.md with private vulnerability reporting policy (GitHub Advisory)
• Add CONTRIBUTING.md with development setup, test commands, PR workflow, and label guide
• Add .github/PULL_REQUEST_TEMPLATE.md with type-of-change checklist
• Add .github/CODEOWNERS for auto-review assignment
• Enable GenerateDocumentationFile in src/Directory.Build.props for NuGet XML API docs
• Expand PackageTags and add PackageReleaseNotes in src/Directory.Build.props

Motivation

Based on open source best practices audit. Key gaps addressed:

Before	After
No security disclosure channel	SECURITY.md → GitHub Private Vulnerability Reporting
No contributor guide	CONTRIBUTING.md with full setup + PR workflow
No PR template	Checklist enforcing label + build + test
No CODEOWNERS	Auto-assigns @akunzai as reviewer
No XML API docs in NuGet package	GenerateDocumentationFile=true (CS1591 suppressed for incremental adoption)

Verification

• dotnet build -c Release → ✅ 0 warnings, 0 errors
• markdownlint-cli2 → ✅ 0 errors
• XML docs confirmed generated for all src/ TFMs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.58%. Comparing base (87f81f0) to head (334ba8d).
⚠️ Report is 28 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #936      +/-   ##
==========================================
+ Coverage   71.08%   72.58%   +1.49%     
==========================================
  Files          42       25      -17     
  Lines        1048      558     -490     
  Branches      136       75      -61     
==========================================
- Hits          745      405     -340     
+ Misses        240      119     -121     
+ Partials       63       34      -29     

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