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-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))