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.
Sets up the initial project structure: two packages (@kitchen-md/core
and @kitchen-md/bin), shared tsconfig, shared fixtures, per-package
test stubs (unit, integration, smoke), .gitignore, and project
CLAUDE.md with Conventional Commits convention.
