refactor(#2693): pass claudeCodePackage straight into serve.json

`builtins.toJSON` already serialises a derivation as its out path and
`null` as `null`, so the `if … then null else "${…}"` binding was doing
by hand what the serialiser does anyway. Hand the package in whole and
drop the intermediate.

The gc-root property is unchanged and re-measured on the real module:
`environment.etc."hyperhive/serve.json".text` still `hasContext`, so the
host system closure still holds the package alive. Verified both ways —
with the package set the rendered JSON is byte-identical to the
interpolated version, and unset still emits `null` — with all module
assertions passing in each case.

The assertion now checks `builtins.toJSON cfg.claudeCodePackage`, which
is the value that actually has to carry the context, rather than an
intermediate that no longer exists.

nix/host-modules/hive-c0re/default.nix

@@ -59,22 +59,6 @@ let
     fi
   '';
 
-  # Store path of the `claude-code` every agent runs, or null for "each
-  # agent keeps the one out of its own nixpkgs". meta.rs writes it into
-  # each agent's generated flake as a plain string literal, and the agent
-  # module puts its `bin/` on the harness PATH.
-  #
-  # This interpolation is also the package's gc root, and the only one:
-  # it carries store context, `builtins.toJSON` preserves that, so the
-  # /etc entry below genuinely references the package and the host's
-  # system closure holds it alive. Nothing on the container side can —
-  # a path spelled out in a generated flake is text, not a reference.
-  # Hence the assertion further down: do NOT discard this context, and
-  # do not hand meta.rs the path by a route that drops it. The failure
-  # mode is a garbage-collected `claude` and a hive that can't take a
-  # turn, weeks after the commit that caused it.
-  claudeCodePath = if cfg.claudeCodePackage == null then null else "${cfg.claudeCodePackage}";
-
   # The `hive-c0re serve` config JSON. Keys are snake_case to match the
   # `ServeConfig` serde shape the daemon deserialises (the
   # container-injected HiveEnv fields, flattened, plus the hive-c0re-local
@@ -92,7 +76,20 @@ let
     hyperhive_flake = cfg.hyperhiveFlake;
     hyperhive_docs_flake = cfg.hyperhiveDocs;
     nixpkgs_flake = cfg.nixpkgsFlake;
-    claude_code_path = claudeCodePath;
+    # The `claude-code` every agent runs, or null for "each agent keeps
+    # the one out of its own nixpkgs". `builtins.toJSON` serialises a
+    # derivation as its out path (and null as null), so the package goes
+    # in whole rather than interpolated — that is also the ONLY thing
+    # gc-rooting it: the resulting string carries store context, so this
+    # /etc entry genuinely references the package and the host's system
+    # closure holds it alive. Nothing container-side can — meta.rs writes
+    # the path into each agent's generated flake as a plain string
+    # literal, and text is not a reference. Hence the assertion below: do
+    # NOT discard this context, and do not route the path through
+    # anything that drops it. The failure mode is a garbage-collected
+    # `claude` and a hive that can't take a turn, weeks after the commit
+    # that caused it.
+    claude_code_path = cfg.claudeCodePackage;
     dashboard_port = cfg.dashboardPort;
     operator_pronouns = cfg.operatorPronouns;
     context_window_tokens = cfg.contextWindowTokens;
@@ -119,7 +116,8 @@ in
         # deploy — it only shows up as every agent failing to spawn
         # `claude`, at whatever unrelated moment the gc runs. Cheap
         # enough to just check.
-        assertion = cfg.claudeCodePackage == null || builtins.hasContext claudeCodePath;
+        assertion =
+          cfg.claudeCodePackage == null || builtins.hasContext (builtins.toJSON cfg.claudeCodePackage);
         message = ''
           services.hyperhive.c0re.claudeCodePackage lost its store
           context on the way into /etc/hyperhive/serve.json, so the