feat: add flake app, dev shell, direnv, and checks (task 0002)

Layer the remaining flake outputs onto the build: a default app pointing at
the compiled binary, a dev shell exposing the pinned Bun, biome, and bun2nix,
a `use flake` .envrc for direnv, and a checks set aggregating the Bun test
tiers, a build-seam smoke check against the Nix-built binary, and biome lint
through the treefmt-nix flake-parts module.

The dev shell and the build share the one pinned nixpkgs-bun input so Bun
cannot drift between them. treefmt's biome reuses the repo's biome.json with
VCS disabled for the sandbox, and packages/core/src/index_test.ts is
reformatted by biome so the new lint gate is green.

.gitignore

@@ -7,6 +7,9 @@ packages/bin/kitchen
 result
 result-*
 
+# direnv / nix-direnv cache
+.direnv/
+
 # local environment overrides
 .env
 .env.local