feat(quality): add markdownlint-cli2 to quality checks (#1039)

- Add markdownlint-cli2 as dev dependency
- Add lint:md script to package.json
- Add markdownlint job to CI workflow
- Configure 5 rules: heading-increment, no-duplicate-heading,
  no-trailing-punctuation, no-bare-urls, no-space-in-emphasis
- Fix existing violations across 19 markdown files
- No auto-fix to prevent destructive changes

Closes #1034

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Brian <bmadcode@gmail.com>

src/core/resources/excalidraw/library-loader.md

@@ -4,7 +4,7 @@
 
 ## Purpose
 
-Load external .excalidrawlib files from https://libraries.excalidraw.com or custom sources.
+Load external .excalidrawlib files from <https://libraries.excalidraw.com> or custom sources.
 
 ## Planned Capabilities
 
@@ -34,7 +34,7 @@ libraries:
 
 ## Implementation Notes
 
-This will be developed when agents need to leverage the extensive library ecosystem available at https://libraries.excalidraw.com.
+This will be developed when agents need to leverage the extensive library ecosystem available at <https://libraries.excalidraw.com>.
 
 Hundreds of pre-built component libraries exist for: