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.

docs/deployment.md

@@ -68,6 +68,10 @@ The shared GUI home-manager module installs Codex from the `llm-agents` input, r
 that input's tested nixpkgs pin. Run `nix flake update llm-agents`, then switch the system
 or home configuration to use the updated package.
 
+The shared cache settings in `lib.my.c.nix.cache` and the flake's `nixConfig` include
+the [Numtide binary cache](https://github.com/numtide/llm-agents.nix#binary-cache), so
+Codex can be downloaded prebuilt. Keep the input's tested nixpkgs pin to preserve cache hits.
+
 `programs.codex.mutableSettings` merges the declared status-line settings into the writable
 `~/.codex/config.toml` during activation. Declared values take precedence on each switch;
 other settings, including model selection, providers and project trust, remain imperative.
@@ -178,8 +182,8 @@ are unchanged, and the Determinate NixOS module is deliberately not imported.
   Determinate without further change.
 - **`accept-flake-config`.** Set true only in the devshell `nix.conf`, `.envrc` (as
   `--accept-flake-config`, for direnv) and CI — the contexts that build this flake — so its
-  `nixConfig` (the Harmonia cache) is trusted without an interactive prompt. It is deliberately not
-  set system-wide: boxes already trust that cache via `nix.settings`, so a global setting would only
+  `nixConfig` (the Harmonia and Numtide caches) is trusted without an interactive prompt. It is
+  deliberately not set system-wide: boxes already trust those caches via `nix.settings`, so a global setting would only
   blanket-trust every flake's `nixConfig` for no gain.
 
 ## Secrets