fix(highlight): complete markdown syntax highlighting with injections

Fix markdown code block highlighting to properly support both markdown
structure (headings, frontmatter) and language injection (rust, bash).

The key fix uses `#set! injection.include-children` in the injection
query to override tree-sitter-md's internal tokenization, enabling
proper highlighting of embedded languages within code fences.

Changes:
- Use crate's HIGHLIGHT_QUERY_BLOCK for base markdown highlighting
- Add custom injection query with include-children directive
- Add YAML/TOML frontmatter and HTML block injection rules
- Add text.* highlight names (title, literal, uri, reference)
- Add string.escape highlight name
- Add CSS styles for new highlight classes
- Remove unused custom md-highlights.scm

queries/md-highlights.scm

@@ -1,35 +0,0 @@
-; 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

@@ -1,7 +1,24 @@
-; Markdown injection queries - testing include-children directive
+; Markdown injection queries with include-children directive
+; Enables proper highlighting of embedded languages in code blocks and frontmatter
 
+; Fenced code blocks - inject language specified in info string
 (fenced_code_block
   (info_string
     (language) @injection.language)
   (code_fence_content) @injection.content
   (#set! injection.include-children))
+
+; YAML frontmatter (--- delimited at start of document)
+((minus_metadata) @injection.content
+  (#set! injection.language "yaml")
+  (#set! injection.include-children))
+
+; TOML frontmatter (+++ delimited)
+((plus_metadata) @injection.content
+  (#set! injection.language "toml")
+  (#set! injection.include-children))
+
+; HTML blocks
+((html_block) @injection.content
+  (#set! injection.language "html")
+  (#set! injection.include-children))