feat(highlight): add markdown syntax highlighting with injection

Add tree-sitter-md for markdown parsing with injection support for
fenced code blocks. Code inside markdown code fences (```rust, ```bash,
etc.) is now fully syntax highlighted.

Key fix: Use `#set! injection.include-children` directive in the
injection query to override tree-sitter-md's internal tokenization
of code_fence_content, allowing proper language injection.

- Add tree-sitter-md v0.5.2 dependency
- Add Markdown variant to Language enum (md, markdown aliases)
- Create queries/md-highlights.scm (minimal markdown highlights)
- Create queries/md-injections.scm (with include-children directive)
- Add test: test_markdown_injection_rust

docs/content/features/syntax-highlighting.md

@@ -12,7 +12,7 @@ sukr highlights code blocks at build time using Tree-sitter. No client-side Java
 
 Use fenced code blocks with a language identifier:
 
-````markdown
+````md
 ```rust
 fn main() {
     println!("Hello, world!");