build: package the kitchen CLI as a Nix flake (task 0001)

Add a flake-parts flake whose default package Bun-compiles the kitchen
CLI into a self-contained native binary, built in the sandbox with
dependencies vendored by bun2nix.

Bun is pinned via a dedicated nixpkgs-bun input (Bun 1.3.13) that
bun2nix follows, so nixpkgs can track unstable without moving the
compile toolchain onto a release that emits empty binaries.

bun2nix is both a flake input and an npm devDependency, and a
postinstall hook keeps the checked-in bun.nix in sync with the lockfile.

.gitignore

@@ -3,6 +3,10 @@ node_modules/
 # compiled CLI binary
 packages/bin/kitchen
 
+# nix build result symlinks
+result
+result-*
+
 # local environment overrides
 .env
 .env.local