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

Cargo.toml

@@ -38,6 +38,7 @@ toml     = "0.8"
 
 # Diagram rendering
 mermaid-rs-renderer = { version = "0.1", default-features = false }
+tree-sitter-md      = "0.5.2"
 
 # Patch dagre_rust to fix unwrap on None bug
 [patch.crates-io]