shell: add env vars for channel store paths

Useful for quickly looking up package and module definitions, e.g:

    diff {$nixos,$nixos_unstable}/pkgs/by-name/ju/jujutsu

2 files changed, 6 insertions, 0 deletions

diff --git a/.envrc b/.envrc
new file mode 100644
index 0000000..03fee3a
--- /dev/null
+++ b/.envrc
@@ -0,0 +1,3 @@
+export nixos="$(npins -d nixos/npins get-path nixos)"
+export nixos_small="$(npins -d nixos/npins get-path nixos-small)"
+export nixos_unstable="$(npins -d nixos/npins get-path nixos-unstable)"
diff --git a/AGENTS.md b/AGENTS.md
index 53c787f..b5b06c6 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -34,6 +34,9 @@ To get the store path to a pin, use the `get-path` npins subcommand:
 
     npins -d nixos/npins get-path <name>
 
+For the Nixpkgs pin store paths `.envrc` defines the env vars `$nixos`,
+`$nixos_small` and `$nixos_unstable`.
+
 Build a package under `nixos/profiles/<profile>/pkgs/`
 (both `<package>.nix` and `<package>/package.nix` map to `<package>`):