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

queries/md-injections.scm

@@ -0,0 +1,7 @@
+; Markdown injection queries - testing include-children directive
+
+(fenced_code_block
+  (info_string
+    (language) @injection.language)
+  (code_fence_content) @injection.content
+  (#set! injection.include-children))