feat(pi): package @earendil-works/pi-coding-agent as pi

Vendors the npm tarball + lockfile and wraps the `pi` binary with `fd` and
`ripgrep` on PATH. Also installs it on the m4 darwin host.

`buildNpmPackage` is pulled from `inputs.unstable` because nixos-25.11's
`prefetch-npm-deps-0.1.0` panics on cacache index entries that contain
either multiple lines or JSON values with embedded spaces (npm's
`accept: application/...; q=1.0, ...` headers). For this lockfile,
`@esbuild/netbsd-arm64` and `@rollup/rollup-linux-x64-musl` trigger
both conditions and `--map-cache` fails with `EOF while parsing a
string at line 1 column 369`. Fixed upstream in nixos-unstable, which
now uses `lines()` + `split_once('\t')`.

packages/pi/hashes.json

@@ -0,0 +1,5 @@
+{
+  "version": "0.74.0",
+  "sourceHash": "sha256-l0pzuWGVvX1jDhFYaey14N16XDo47kkm3JlEhmPUo0Q=",
+  "npmDepsHash": "sha256-KQsuZzH8bdnvyD/ZcSIZyDPO4th6+osbXFZmaK21eOc="
+}