feat(#2170): split docs/ into its own meta-flake input

nix/modules/hive-c0re.nix

@@ -3,6 +3,7 @@
   hyperhiveFrontend,
   hyperhiveAssets,
   hyperhiveFlake,
+  hyperhiveDocs,
   agentBaseToplevel,
   managerToplevel,
 }:
@@ -39,6 +40,7 @@ let
   serveConfig = pkgs.writeText "hive-c0re-serve.json" (
     builtins.toJSON {
       hyperhive_flake = cfg.hyperhiveFlake;
+      hyperhive_docs_flake = cfg.hyperhiveDocs;
       nixpkgs_flake = cfg.nixpkgsFlake;
       dashboard_port = cfg.dashboardPort;
       operator_pronouns = cfg.operatorPronouns;
@@ -558,6 +560,19 @@ in
         only override if you want agents tracking a different ref.
       '';
     };
+    hyperhiveDocs = lib.mkOption {
+      type = lib.types.str;
+      default = hyperhiveDocs;
+      defaultText = lib.literalMD "the docs/ tree's own store path";
+      description = ''
+        URL of the narrow `docs/` source (no fragment). Inlined into the
+        generated meta `flake.nix` at `inputs.hyperhive-docs.url` and
+        threaded to each agent as `hyperhive.docs.source`, from which the
+        harness resolves `$HIVE_DOCS_DIR`. Its own store path — separate
+        from `hyperhiveFlake` — so a doc edit only re-locks this input
+        instead of rebuilding every agent container.
+      '';
+    };
     nixpkgsFlake = lib.mkOption {
       type = lib.types.str;
       default = "path:${pkgs.path}";