build(packages): bump llama-cpp to b9159, add WebUI derivation, fix spec-type

- Bump llama-cpp from b9048 to b9159
- Add WebUI build derivation to work around HF bucket fetch in Nix sandbox
- Switch MTP patch from .patch to .diff (squashed unified diff applies cleanly)
- Refactor default.nix with let bindings for cleaner structure
- Add AGENTS.md documenting version/postFetch pitfalls
- Add qwen3.6-27b-vllm-50k single-GPU config to llama-swap
- Fix --spec-type from "mtp" to "draft-mtp" in llama.cpp configs
- Update update-package-hashes skill with fetchpatch/.diff guidance

packages/llama-cpp/default.nix

@@ -1,4 +1,75 @@
 { pkgs }:
+let
+  version = "9159";
+
+  src = pkgs.fetchFromGitHub {
+    owner = "ggml-org";
+    repo = "llama.cpp";
+    tag = "b${version}";
+    hash = "sha256-y69ZmVFxo7bQvLTT6/GWwkb5j4Ll8eXSVXFpfXVkvyg=";
+    leaveDotGit = true;
+    postFetch = ''
+      git -C "$out" rev-parse --short HEAD > $out/COMMIT
+      find "$out" -name .git -print0 | xargs -0 rm -rf
+    '';
+  };
+
+  # MTP Patch (PR #22673)
+  # Use the .diff (squashed) endpoint, not .patch (mbox of commits).
+  mtpPatch = pkgs.fetchpatch {
+    name = "mtp.patch";
+    url = "https://github.com/ggml-org/llama.cpp/pull/22673.diff";
+    hash = "sha256-8W02V7oqq2/SzSrDUHEa5Zm+dGBFkasOiVinww3V85U=";
+  };
+
+  # Pre-Built WebUI Assets
+  # As of b9151 llama.cpp removed the prebuilt WebUI from the repo and tries to
+  # curl them from a HuggingFace bucket at build time. That fails in the Nix
+  # sandbox. We build the WebUI from source in a separate derivation and drop
+  # the 4 output files into tools/server/public/ so cmake's "Priority 1: local
+  # assets present" branch short-circuits the network fetch.
+  # Vendored npm deps. The npmDeps FOD uses default unpack and looks for
+  # package-lock.json at sourceRoot, so point it directly at the webui dir.
+  webuiNpmDeps = pkgs.fetchNpmDeps {
+    name = "llama-webui-${version}-npm-deps";
+    inherit src;
+    sourceRoot = "${src.name}/tools/server/webui";
+    hash = "sha256-WaEePrEZ7O/7deP2KJhe0AwiSKYA8HOqETmMHUkmBe0=";
+  };
+
+  webui = pkgs.buildNpmPackage {
+    pname = "llama-webui";
+    inherit version src;
+
+    # Custom unpack: the vite plugin writes to ../public, so both webui/ and
+    # public/ must be writable siblings under tools/server/. Plain sourceRoot
+    # leaves the parent dirs in the read-only Nix store.
+    unpackPhase = ''
+      runHook preUnpack
+      cp -r ${src}/tools/server tools-server
+      chmod -R u+w tools-server
+      cd tools-server/webui
+      runHook postUnpack
+    '';
+
+    npmDeps = webuiNpmDeps;
+
+    # The vite plugin writes to ../public; ensure it exists.
+    preBuild = ''
+      mkdir -p ../public
+    '';
+
+    installPhase = ''
+      runHook preInstall
+      mkdir -p $out
+      install -Dm644 ../public/index.html   $out/index.html
+      install -Dm644 ../public/bundle.js    $out/bundle.js
+      install -Dm644 ../public/bundle.css   $out/bundle.css
+      install -Dm644 ../public/loading.html $out/loading.html
+      runHook postInstall
+    '';
+  };
+in
 (pkgs.llama-cpp.override {
   cudaSupport = true;
   blasSupport = true;
@@ -6,19 +77,8 @@
   metalSupport = false;
   vulkanSupport = true;
 }).overrideAttrs
-  (oldAttrs: rec {
-    version = "9048";
-    src = pkgs.fetchFromGitHub {
-      owner = "ggml-org";
-      repo = "llama.cpp";
-      tag = "b${version}";
-      hash = "sha256-lYtX0hLReCnFw1+xOKefly+WunuoN89ZFEFl5mK5pQ4=";
-      leaveDotGit = true;
-      postFetch = ''
-        git -C "$out" rev-parse --short HEAD > $out/COMMIT
-        find "$out" -name .git -print0 | xargs -0 rm -rf
-      '';
-    };
+  (oldAttrs: {
+    inherit version src;
 
     # Add SPIR-V Headers for Vulkan Backend
     # Newer llama.cpp requires spirv/unified1/spirv.hpp which isn't
@@ -39,11 +99,17 @@
 
     # Apply Patches
     patchFlags = [ "-p1" ];
-    patches = (oldAttrs.patches or [ ]) ++ [
-      (pkgs.fetchpatch {
-        name = "mtp.patch";
-        url = "https://github.com/ggml-org/llama.cpp/pull/22673.patch";
-        hash = "sha256-HqpchhOpxuw5mY4a/OCWGDr2Y32rC4FeOHuhaVt+mvY=";
-      })
-    ];
+    patches = (oldAttrs.patches or [ ]) ++ [ mtpPatch ];
+
+    # Drop pre-built WebUI assets into tools/server/public/ so cmake's
+    # Priority 1 path picks them up and skips the HF Bucket fetch.
+    postPatch = ''
+      ${oldAttrs.postPatch or ""}
+      mkdir -p tools/server/public
+      cp ${webui}/* tools/server/public/
+    '';
+
+    # Expose the WebUI sub-derivation so it can be built/tested in isolation:
+    #   nix build .#llama-cpp.webui --builders ''
+    passthru = (oldAttrs.passthru or { }) // { inherit webui; };
   })