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-highlights.scm

@@ -0,0 +1,35 @@
+; Minimal markdown block highlighting - only capture outside code blocks
+; Removed all captures that could match inside code_fence_content
+
+(atx_heading (inline) @text.title)
+(setext_heading (paragraph) @text.title)
+
+[
+  (atx_h1_marker)
+  (atx_h2_marker)
+  (atx_h3_marker)
+  (atx_h4_marker)
+  (atx_h5_marker)
+  (atx_h6_marker)
+  (setext_h1_underline)
+  (setext_h2_underline)
+] @punctuation.special
+
+(fenced_code_block_delimiter) @punctuation.delimiter
+(info_string (language) @string)
+
+(link_title) @string
+(link_destination) @text.uri
+(link_label) @text.reference
+
+[
+  (list_marker_plus)
+  (list_marker_minus)
+  (list_marker_star)
+  (list_marker_dot)
+  (list_marker_parenthesis)
+  (thematic_break)
+] @punctuation.special
+
+(block_quote_marker) @punctuation.special
+(backslash_escape) @string.escape

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))