chore: scaffold Bun workspace monorepo

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.

.gitignore

@@ -0,0 +1,8 @@
+node_modules/
+
+# compiled CLI binary
+packages/bin/kitchen
+
+# local environment overrides
+.env
+.env.local