Add AGENTS.md documentation and automated quality checks for markdown

[Copilot]

Adds repository documentation for GitHub Copilot agents and enforces spelling/markdown standards via CI.

New Files

• AGENTS.md - Repository structure, development guidelines, and agent instructions
• .cspell.json - Spelling dictionary with project-specific terms (Dotnet, nuspec, SBOM, etc.)
• .markdownlint.json - Markdown rules: 120 char line length, HTML allowed, code blocks exempt

Workflow Changes

Added quality-checks job to build_on_push.yaml that gates the build:

quality-checks:
  name: Quality Checks
  runs-on: ubuntu-latest
  permissions:
    contents: read
  steps:
    - name: Run Spelling Check
      run: npx cspell "**/*.md"
    - name: Run Markdown Linting
      run: npx markdownlint-cli "**/*.md"

build:
  needs: quality-checks
  # ... existing build steps

Fixes

• Wrapped long lines in README.md to meet 120 char limit
• Added language specifier to fenced code block
• Added node_modules to .gitignore

Original prompt

I would like to add an AGENTS.md file to this repository based on its current contents, and also adding .cspell.json and .markdownlint.json files to enforce spelling and markdown formatting. The spelling and markdown formatting should be performed by adding a new "Quality Checks" job to the build_on_push.yaml workflow file that runs the node cspell and markdownlint-cli tools.

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