1. NEW: extract_markdown() — structurally indexes .md/.mdx files into the
knowledge graph. Headings become nodes, code blocks become nodes with
language tags, and nesting produces 'contains' edges. Zero new deps
(pure regex/line-by-line parsing, no tree-sitter needed).
2. FIX: collect_files() _EXTENSIONS was hardcoded and missing 18 extensions
that _DISPATCH already supported (.jsx, .mjs, .ex, .exs, .jl, .vue,
.svelte, .dart, .v, .sv, .sql, .f, .F, .f90, etc). Now uses
set(_DISPATCH.keys()) to stay automatically in sync.
3. Added deploy_guide.md test fixture and 6 new test cases.
4. Updated test_collect_files_from_dir to use dynamic extension set.
Farhan M