fix(#2213): decouple docs drvs from self store path via narrow nixSrc

The docs evaluation referenced self.nixosConfigurations.agent-base and
self.nixosModules.default — both tied to self's full flake store path.
Every commit produces a new self hash (even when only Rust or frontend
files change), so docs drvs change on every commit and the remote
builder (muede-pc2) must rebuild them from scratch each time.

Fix: use builtins.path on the nix/ directory with
builtins.unsafeDiscardStringContext to produce a content-addressed store
path (nixSrc) that only changes when .nix files change. Evaluate both
hostEval and agentEval from nixSrc using stub package args (pkgs.emptyFile /
pkgs.emptyDirectory) instead of self.overlays.default, which avoids
pulling in the Rust and frontend build closures.

Now docs drvs only change when a .nix file changes. PRs that touch only
Rust or frontend code hit the remote builder cache instead of forcing a
rebuild. Closes the disk-pressure-induced CI failure loop on muede-pc2.

Forge URLs in transformOptions updated to prepend nix/ prefix correctly
(nixSrc is a copy of nix/, so stripped paths need nix/ re-added).

nix/docs/default.nix

@@ -11,17 +11,50 @@
 # prose `/docs/` tree. Full pipeline + subtree-pick / output-tree
 # rationale: docs/gotchas.md::Nix options reference.
 let
+  # Content-addressed narrow source covering only the nix/ directory.
+  # `builtins.unsafeDiscardStringContext` strips `self`'s store-path
+  # context so `builtins.path` hashes only the nix/ file content, not
+  # the full flake source (Rust, frontend, markdown, …).  Result: the
+  # docs drvs only change when a .nix file changes, not on every
+  # commit — the muede-pc2 remote builder can reuse its cached result
+  # for any commit that doesn't touch nix/.
+  nixSrc = builtins.path {
+    path = builtins.unsafeDiscardStringContext (toString self + "/nix");
+    name = "hyperhive-nix-src";
+  };
+
+  # Stub overlay that satisfies pkgs.hyperhive-* references in module
+  # option defaults without depending on self's Rust / frontend builds.
+  # nixosOptionsDoc renders `defaultText` for these options anyway; the
+  # stubs just prevent attribute-missing eval errors.
+  docsStubOverlay = _final: _prev: {
+    hyperhive = pkgs.emptyFile;
+    hyperhive-frontend = pkgs.emptyDirectory;
+    hyperhive-assets = pkgs.emptyDirectory;
+    hyperhive-docs = pkgs.emptyDirectory;
+  };
+
   # Stub host system: every hyperhive subsystem `mkForce false` so
   # heavy build inputs (matrix container, forge, etc.) stay out of
   # the eval — only option *declarations* matter for the doc walk.
+  # Import hive-c0re.nix from the content-addressed nixSrc with stub
+  # package args so the eval doesn't depend on self's Rust builds.
   hostEval = nixosSystem {
     system = pkgs.stdenv.hostPlatform.system;
     modules = [
-      self.nixosModules.default
+      (import "${nixSrc}/modules/hive-c0re.nix" {
+        hyperhivePackage = _system: pkgs.emptyFile;
+        hyperhiveFrontend = _system: pkgs.emptyFile;
+        hyperhiveAssets = _system: pkgs.emptyDirectory;
+        hyperhiveFlake = "";
+        hyperhiveDocs = "";
+        agentBaseToplevel = pkgs.emptyFile;
+        managerToplevel = pkgs.emptyFile;
+      })
       (
         { lib, ... }:
         {
-          nixpkgs.overlays = [ self.overlays.default ];
+          nixpkgs.overlays = [ docsStubOverlay ];
           fileSystems."/" = {
             device = "/dev/null";
             fsType = "tmpfs";
@@ -35,14 +68,23 @@ let
     ];
   };
 
-  # Reuse the already-evaluated agent-base config — its options tree is
-  # identical to what a real agent container sees, no second eval needed.
-  agentEval = self.nixosConfigurations.agent-base;
+  # Agent module eval from the content-addressed nixSrc.  Relative
+  # imports inside agent-base.nix (e.g. ./harness-base.nix) resolve
+  # correctly against the nixSrc directory tree.
+  agentEval = nixosSystem {
+    system = pkgs.stdenv.hostPlatform.system;
+    modules = [
+      "${nixSrc}/templates/agent-base.nix"
+      { nixpkgs.overlays = [ docsStubOverlay ]; }
+    ];
+  };
 
   # Rewrite option declaration paths from nix-store absolute paths to
   # forge URLs so rendered docs link back to source.
+  # nixSrc is a content-addressed copy of nix/; strip its store prefix
+  # and prepend nix/ to recover the repo-relative path.
   forgeRoot = "https://forge.darkest.space/hyperhive/hyperhive/src/branch/main";
-  storePrefix = toString self + "/";
+  nixSrcPrefix = builtins.unsafeDiscardStringContext (toString nixSrc + "/");
   transformOptions =
     opt:
     opt
@@ -52,8 +94,8 @@ let
         let
           declStr = toString decl;
           relPath =
-            if lib.hasPrefix storePrefix declStr then
-              lib.removePrefix storePrefix declStr
+            if lib.hasPrefix nixSrcPrefix declStr then
+              "nix/" + lib.removePrefix nixSrcPrefix declStr
             else
               baseNameOf declStr;
         in
@@ -136,7 +178,7 @@ let
       `hyperhive.forge.*`, `hyperhive.matrix.*`, `hyperhive.gui.*`).
 
     Regenerate with `nix build .#docs` (bundle), `.#docs-host`, or
-    `.#docs-agent`.
+    `.#docs-agent`. Consumed by the website repo to render `/options/`.
   '';
 
   hostMD = mkMarkdownPage "docs-host" "hyperhive — host options" hostDoc;