feat(dogfood): route .mts/.cts to typescript + .mdx to markdown

Dogfood (PR #142 1B + multi-root: kebab-docs + httpx + zod + lodash)
showed 28 files skipped by extension that are routable to existing
extractors:
- .mts (ESM TypeScript) / .cts (CommonJS TypeScript) — same grammar as
  .ts in tree-sitter-typescript 0.23 (LANGUAGE_TYPESCRIPT covers JSX-
  agnostic variants; LANGUAGE_TSX stays for .tsx only)
- .mdx (Markdown + JSX) — routed as MediaType::Markdown; the md parser
  folds JSX islands through as raw passthrough

Changes:
- crates/kebab-source-fs/src/media.rs: 'mts'|'cts' → Code(typescript),
  'mdx' → Markdown. +2 unit tests.
- crates/kebab-parse-code/src/lang.rs: code_lang_for_path matches mts/cts;
  module_path_for_tsjs strips .mts/.cts as well. Test cases extended.
- crates/kebab-parse-code/src/typescript.rs: doc comment on select_grammar
  refreshed to mention .mts/.cts.
- crates/kebab-parse-code/tests/lang.rs: 2 new assertions.

verify: kebab-source-fs 44 / kebab-parse-code lib 20 + lang 4 all pass; clippy clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-parse-code/tests/lang.rs

@@ -9,6 +9,8 @@ fn known_extensions_map_to_canonical_identifiers() {
         ("foo.pyi", Some("python")),
         ("foo.ts", Some("typescript")),
         ("foo.tsx", Some("typescript")),
+        ("foo.mts", Some("typescript")),  // ESM TS — same grammar
+        ("foo.cts", Some("typescript")),  // CommonJS TS — same grammar
         ("foo.js", Some("javascript")),
         ("foo.mjs", Some("javascript")),
         ("foo.cjs", Some("javascript")),