feat: render richer blocks and inline nodes (task 0004)

Extend the core parser and the CLI renderer to cover the everyday Markdown
body beyond headings and paragraphs: lists, blockquotes, fenced code,
thematic breaks, and inline emphasis, strong, code spans, and links.

Add remark-gfm to the pipeline so tables and strikethrough parse as their
own nodes. Any block or inline node core does not model falls through to
RawBlock / RawInline, whose verbatim value is position-sliced from the
original input so unmodelled constructs round-trip byte-for-byte.

packages/core/package.json

@@ -11,6 +11,7 @@
   },
   "dependencies": {
     "remark-frontmatter": "^5.0.0",
+    "remark-gfm": "^4.0.1",
     "remark-parse": "^11.0.0",
     "unified": "^11.0.5",
     "yaml": "^2.9.0"