nix: Add Numtide binary cache for llm-agents

Configure the Numtide substituter and signing key in the shared cache
settings and flake configuration so Codex can use prebuilt binaries.
Document the cache and retain the input’s tested nixpkgs pin.

flake.nix

@@ -1,16 +1,18 @@
 {
   description = "System configs";
 
-  # Offer our Harmonia cache when building the flake itself, so `nix develop` / `nix build` don't
+  # Offer our Harmonia and Numtide caches when building the flake, so `nix develop` / `nix build` don't
   # rebuild from source. Nix reads `nixConfig` before the flake evaluates and rejects any computed
   # value (imports/thunks), so these must stay literal — keep them in sync with `lib.my.c.nix.cache`.
   # Consumers must trust these (accept-flake-config / a trusted user) for them to take effect.
   nixConfig = {
     extra-substituters = [
       "https://nix-cache.nul.ie"
+      "https://cache.numtide.com"
     ];
     extra-trusted-public-keys = [
       "nix-cache.nul.ie-1:BzH5yMfF4HbzY1C977XzOxoPhEc9Zbu39ftPkUbH+m4="
+      "niks3.numtide.com-1:DTx8wZduET09hRmMtKdQDxNNthLQETkc/yaX7M4qK0g="
     ];
   };