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.lock

@@ -1531,6 +1531,7 @@ dependencies = [
  "tree-sitter-html",
  "tree-sitter-javascript",
  "tree-sitter-json",
+ "tree-sitter-md",
  "tree-sitter-nix",
  "tree-sitter-python",
  "tree-sitter-rust",
@@ -1809,6 +1810,16 @@ version = "0.1.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "4ae62f7eae5eb549c71b76658648b72cc6111f2d87d24a1e31fa907f4943e3ce"
 
+[[package]]
+name = "tree-sitter-md"
+version = "0.5.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2c96068626225a758ddb1f7cfb82c7c1fab4e093dd3bde464e2a44e8341f58f5"
+dependencies = [
+ "cc",
+ "tree-sitter-language",
+]
+
 [[package]]
 name = "tree-sitter-nix"
 version = "0.3.0"