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, 4 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 f2776d5..a50e5f3 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -12,6 +12,7 @@ Dependencies are pinned with npins; host-to-channel mappings are defined in `nix
 
 * To inspect and edit pins use the `npins` CLI: `npins -d nixos/npins <command>`.
 * To get the store path to a pin, use the `get-path` npins subcommand.
+  For the Nixpkgs pin store paths `.envrc` defines the env vars `$nixos`, `$nixos_small`, `$nixos_unstable`.
 * Avoid `nixos-option`.
 
 ## Documentation