agent-modules: hand an api-key agent's subagent daemon the backend file

A claude agent with useApiKey gets ANTHROPIC_API_KEY through
backendEnvironmentFile, but only the harness unit loaded that file;
hive-subagent-daemon loaded it for ACP agents alone. The daemon's nested
claude inherits the daemon's environment (hive-claude adds to it, never
clears it), and an api-key agent has no OAuth session in ~/.claude, so
its claude subagents started with no credentials.

hive-subagent-daemon now loads backendEnvironmentFile when the agent is
ACP (unchanged) or has useApiKey set. Module-eval covers the api-key
claude arm; the existing claude arm (file set, useApiKey unset) still
asserts no EnvironmentFile.

Closes #5088

docs/turn-loop/config.md

@@ -269,7 +269,9 @@ is a no-op without the other:
   reads from the environment — typically `ANTHROPIC_API_KEY` and
   `ANTHROPIC_BASE_URL`. Loaded as an _optional_ `EnvironmentFile`
   (leading `-`), so setting the option before the file exists doesn't
-  strand the harness at boot.
+  strand the harness at boot. With `useApiKey` set, the subagent daemon
+  (`hive-subagent-daemon`) loads the same file, so the agent's claude
+  subagents run on the same key.
 
 Provision the file once, out of band (never through nix — an API key in
 the store is world-readable and travels with the flake closure):

nix/agent-modules/agent-service.nix

@@ -209,7 +209,9 @@ in
       before creating the file, or a fresh host rebuild before state is
       restored — makes systemd skip it rather than refuse to start the
       harness. See `services.hyperhive.agent.useApiKey`'s doc for the option this one is
-      paired with.
+      paired with. The subagent daemon loads the same file on an ACP agent
+      and on an agent with `useApiKey` set, so its subagents authenticate
+      the same way the harness does.
 
       On an ACP agent using the `opencode` preset, a provider key this file
       leaves unset is read from the swarm secret store at

nix/agent-modules/mcp.nix

@@ -494,13 +494,14 @@ in
       //
         lib.optionalAttrs
           (
-            config.services.hyperhive.agent.runtime == "acp"
+            (config.services.hyperhive.agent.runtime == "acp" || config.services.hyperhive.agent.useApiKey)
             && config.services.hyperhive.agent.backendEnvironmentFile != null
           )
           {
-            # An ACP subagent authenticates to its provider with the same
+            # A subagent authenticates with the same credentials as the
-            # credentials as the harness's ACP agent. Claude subagents are
+            # harness: an ACP subagent to its provider, a claude subagent of
-            # not handed this file.
+            # an api-key agent with `ANTHROPIC_API_KEY`, as it has no OAuth
+            # session to fall back on.
             EnvironmentFile = "-${config.services.hyperhive.agent.backendEnvironmentFile}";
           };
     };

nix/module-eval/agent-runtime.nix

@@ -38,6 +38,13 @@ let
 
   claude = agentOn "claude";
   acp = agentOn "acp";
+  claudeApiKey = agentWith {
+    services.hyperhive.agent = {
+      runtime = "claude";
+      useApiKey = true;
+      backendEnvironmentFile = backendEnv;
+    };
+  };
 
   # The opencode preset, with and without a secret store.
   opencodeWith =
@@ -148,9 +155,15 @@ let
       name = "an ACP agent's subagent daemon loads the backend credentials";
       ok = (subagent acp).serviceConfig.EnvironmentFile or null == "-${backendEnv}";
     }
+    {
+      # The nested `claude` inherits the daemon's environment, so this file
+      # is where its `ANTHROPIC_API_KEY` comes from.
+      name = "an api-key claude agent's subagent daemon loads the backend credentials";
+      ok = (subagent claudeApiKey).serviceConfig.EnvironmentFile or null == "-${backendEnv}";
+    }
     {
       # Unset is what `RuntimeSpec` reads as claude, and a claude subagent
-      # is not handed the backend file.
+      # of an agent without `useApiKey` is not handed the backend file.
       name = "a claude agent's subagent daemon has no runtime selection and no backend file";
       ok =
         lib.all (var: (subagent claude).environment.${var} or null == null) runtimeVars