feat(core): adopt remark/unified as the internal Markdown parser (ADR 0002)

Add unified + remark-parse as dependencies of @kitchen-md/core and wire a
private internal processor (packages/core/src/markdown.ts) that parses
CommonMark into mdast. remark stays an implementation detail and is not
re-exported from the package entry, so consumers take no transitive remark
dependency (ADR 0003).

The OFM wikilink plugin and the public parse/AST surface are deferred to the
core-parser spec: canonical remark-wiki-link@2.0.1 targets an old micromark
generation incompatible with remark-parse@11, and OFM plus annotation
handling is that spec's scope.

This also exercises the flake's bun2nix vendoring against a real dependency
closure (58 vendored packages); nix build still compiles a working binary
offline in the sandbox.

packages/core/package.json

@@ -8,5 +8,12 @@
   },
   "scripts": {
     "test": "bun test"
+  },
+  "dependencies": {
+    "remark-parse": "^11.0.0",
+    "unified": "^11.0.5"
+  },
+  "devDependencies": {
+    "@types/mdast": "^4.0.4"
   }
 }