Add comprehensive GitHub Copilot instructions for dotnet/core repository (#10029)

* Initial plan

* Initial exploration and plan for Copilot instructions

Co-authored-by: jongalloway <68539+jongalloway@users.noreply.github.com>

* Create comprehensive GitHub Copilot instructions for dotnet/core repository

Co-authored-by: jongalloway <68539+jongalloway@users.noreply.github.com>

* Fix JSON_PRETTIER formatting error in markdown-link-check-config.json

Co-authored-by: jongalloway <68539+jongalloway@users.noreply.github.com>

---------

Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jongalloway <68539+jongalloway@users.noreply.github.com>
Co-authored-by: Jon Galloway <jongalloway@gmail.com>
Co-authored-by: James Montemagno <james.montemagno@gmail.com>

.github/copilot-instructions.md

@@ -0,0 +1,174 @@
+# .NET Core Repository (dotnet/core)
+
+Always reference these instructions first and fallback to search or bash commands only when you encounter unexpected information that does not match the info here.
+
+This is the official .NET release notes and announcements repository. It is NOT a buildable codebase repository - it contains documentation, release notes, and metadata about .NET releases. You do not build applications here; you maintain release documentation and validate content.
+
+## Working Effectively
+
+### Initial Setup
+
+Set up the validation tools needed for this documentation repository:
+
+```bash
+npm install markdownlint-cli markdown-link-check prettier
+pip install checkov
+```
+
+### Validation Commands
+
+Always run these validation commands before committing changes. All commands have been tested and work correctly:
+
+1. **Markdown linting** (takes ~1 second):
+
+   ```bash
+   npx markdownlint --config .github/linters/.markdown-lint.yml *.md
+   npx markdownlint --config .github/linters/.markdown-lint.yml release-notes/**/*.md
+   ```
+
+2. **Link checking** (takes 30-180 seconds depending on external links - NEVER CANCEL):
+
+   ```bash
+   npx markdown-link-check --config .github/workflows/markdown-link-check-config.json README.md
+   npx markdown-link-check --config .github/workflows/markdown-link-check-config.json [specific-file].md
+   ```
+
+   TIMEOUT WARNING: Set timeout to 5+ minutes for link checking. External links may be slow or blocked by network restrictions.
+
+3. **Security scanning with checkov** (takes ~38 seconds):
+
+   ```bash
+   checkov --config-file .checkov.yml --directory . --quiet
+   ```
+
+4. **Format checking with prettier** (takes ~5 seconds):
+
+   ```bash
+   npx prettier --check --ignore-path .prettierignore .
+   ```
+
+   Note: This will show format warnings for most files but will not fail. Format fixing is typically not required unless specifically requested.
+
+### Git Operations
+
+Standard git workflow for documentation changes:
+
+```bash
+git --no-pager status
+git --no-pager diff
+git add [specific-files]
+git commit -m "Descriptive commit message"
+git push origin [branch-name]
+```
+
+## Repository Structure
+
+### Key Directories
+
+- `/release-notes/` - Version-specific release notes organized by major version (8.0/, 9.0/, 10.0/, etc.)
+  - Each version has `releases.json`, `supported-os.json`, and markdown files for specific releases
+  - `/release-notes/schemas/` - JSON schemas for release metadata
+  - `/release-notes/templates/` - Templates for new release notes
+- `/Documentation/` - General documentation about .NET Core ecosystem and policies
+- `/.github/workflows/` - GitHub Actions for validation (super-linter, markdown-link-check, etc.)
+- Root level - Policy documents, README, CONTRIBUTING, and support information
+
+### Important Files
+
+- `README.md` - Main repository landing page with current release information
+- `releases.md` - Release schedule and support information
+- `release-policies.md` - .NET release and support policies
+- `release-notes/releases-index.json` - Master index of all .NET releases
+- `CONTRIBUTING.md` - Links to actual contribution guidelines in dotnet/runtime
+
+## Common Tasks
+
+### Adding New Release Notes
+
+1. Navigate to appropriate version directory in `/release-notes/[version]/`
+2. Create new markdown file following existing naming convention (e.g., `9.0.8.md`)
+3. Update `releases.json` with new release metadata
+4. Update root `README.md` if this is a latest patch version
+5. Run all validation commands
+6. Check that links work and markdown is properly formatted
+
+### Updating Release Metadata
+
+Release metadata is stored in JSON files:
+
+- `release-notes/releases-index.json` - Master index
+- `release-notes/[version]/releases.json` - Version-specific releases
+- `release-notes/[version]/supported-os.json` - Supported operating systems
+
+Always validate JSON syntax and schema compliance after edits.
+
+### Updating Documentation
+
+1. Edit markdown files in `/Documentation/` or root level
+2. Run markdown linting: `npx markdownlint --config .github/linters/.markdown-lint.yml [file]`
+3. Check links: `npx markdown-link-check --config .github/workflows/markdown-link-check-config.json [file]`
+4. Verify changes don't break existing navigation or references
+
+## Validation Requirements
+
+### Before Every Commit
+
+Always run these validation steps in order:
+
+1. Markdown linting (must pass without errors)
+2. Link checking (external links may fail due to network restrictions - this is acceptable)
+3. Security scanning with checkov (must pass)
+4. Format checking (warnings are acceptable)
+
+### Manual Validation Scenarios
+
+Since this is a documentation repository, manual validation involves:
+
+1. **Link verification**: Navigate to updated pages and verify internal links work
+2. **Content accuracy**: Ensure release notes match actual .NET releases
+3. **JSON schema validation**: Ensure metadata files follow required schema
+4. **Cross-reference checking**: Verify links between release notes and main README are consistent
+
+### CI/CD Integration
+
+The repository uses GitHub Actions workflows:
+
+- `.github/workflows/super-linter.yml` - Runs comprehensive linting
+- `.github/workflows/markdown-link-check.yml` - Validates markdown links
+- All other workflow files are for issue/PR labeling automation
+
+## Timing Expectations
+
+| Command               | Expected Time  | Timeout Setting |
+| --------------------- | -------------- | --------------- |
+| markdownlint          | 1 second       | 30 seconds      |
+| markdown-link-check   | 30-180 seconds | 5+ minutes      |
+| checkov security scan | 38 seconds     | 2 minutes       |
+| prettier format check | 5 seconds      | 30 seconds      |
+
+NEVER CANCEL these commands early. Network-dependent operations like link checking may take longer than expected.
+
+## Troubleshooting
+
+### Common Issues
+
+1. **JSON syntax errors**: Use a JSON validator before committing changes to .json files
+2. **Broken internal links**: Verify relative paths are correct and files exist
+3. **External link failures**: These are often due to network restrictions and may be acceptable
+4. **Markdown formatting**: Follow existing patterns in similar files
+5. **Schema validation failures**: Ensure JSON metadata follows schemas in `/release-notes/schemas/`
+
+### Network Restrictions
+
+Some external links may fail during link checking due to network restrictions in CI environments. This is expected and acceptable as long as internal links work correctly.
+
+## Related Repositories
+
+This repository links to many other .NET repositories:
+
+- `dotnet/runtime` - Core .NET runtime and libraries
+- `dotnet/sdk` - .NET SDK
+- `dotnet/aspnetcore` - ASP.NET Core
+- See `Documentation/core-repos.md` for complete list
+
+Do not attempt to build, run, or test code here. This repository contains only documentation and metadata about the .NET ecosystem.

.github/workflows/markdown-link-check-config.json

@@ -1,11 +1,11 @@
-{
+{
-  "ignorePatterns": [
+  "ignorePatterns": [
-    {
+    {
-      "pattern": "^http://localhost"
+      "pattern": "^http://localhost"
-    },
+    },
-    {
+    {
-      "pattern": "^release-notes/templates/" // ignore links in docs directory
+      "pattern": "^release-notes/templates/"
-    }
+    }
-  ],
+  ],
-  "aliveStatusCodes": [429, 200]
+  "aliveStatusCodes": [429, 200]
-}
+}

.gitignore

@@ -42,3 +42,8 @@ bld/
 msbuild.log
 msbuild.err
 msbuild.wrn
+
+# Node.js build artifacts
+node_modules/
+package-lock.json
+package.json