feat(ci): add markdownlint-cli2 for consistent markdown formatting

Add automated markdown linting to ensure consistent formatting across
all markdown files in the repository.

Changes:
- Add .markdownlint-cli2.jsonc configuration
- Create .github/workflows/lint.yml for CI/CD integration
- Fix all 908 existing markdown errors across 27 files
- Enforce ATX-style headings and asterisk emphasis
- Set consistent 2-space list indentation

This prevents markdown errors after project initialization and
maintains high documentation quality standards.

docs/installation.md

@@ -42,11 +42,13 @@ uvx --from git+https://github.com/github/spec-kit.git specify init <project_name
 All automation scripts now have both Bash (`.sh`) and PowerShell (`.ps1`) variants.
 
 Auto behavior:
+
 - Windows default: `ps`
 - Other OS default: `sh`
 - Interactive mode: you'll be prompted unless you pass `--script`
 
 Force a specific script type:
+
 ```bash
 uvx --from git+https://github.com/github/spec-kit.git specify init <project_name> --script sh
 uvx --from git+https://github.com/github/spec-kit.git specify init <project_name> --script ps
@@ -63,6 +65,7 @@ uvx --from git+https://github.com/github/spec-kit.git specify init <project_name
 ## Verification
 
 After initialization, you should see the following commands available in your AI agent:
+
 - `/speckit.specify` - Create specifications
 - `/speckit.plan` - Generate implementation plans  
 - `/speckit.tasks` - Break down into actionable tasks
@@ -87,4 +90,3 @@ git config --global credential.helper manager
 echo "Cleaning up..."
 rm gcm-linux_amd64.2.6.1.deb
 ```
-