pi: move extensions into custom-extensions/, register via settings.extensions

Relocate the bundled extensions out of extensions/ into custom-extensions/
and pull their packaging out of default.nix into a focused extensions.nix
module.

Register them through programs.pi-coding-agent.settings.extensions as a
buildNpmPackage derivation, pointing pi directly at the nix-store-backed
path, rather than symlinking into place with home.file.

Assisted-by: pi (claude-opus-4-8)

home/gabriel/features/pi/default.nix

@@ -1,6 +1,7 @@
-{pkgs, ...}: {
+{
   imports = [
     ./theme.nix
+    ./extensions.nix
   ];
 
   programs.pi-coding-agent = {
@@ -22,42 +23,11 @@
       ];
       enableInstallTelemetry = false;
       enableAnalytics = false;
-      gondolin = {
-        httpProxy = {
-          allowedHosts = ["api.github.com"  "firefly.m7.rs"];
-          secrets = {
-            GITHUB_TOKEN = {
-              hosts = ["api.github.com"];
-              cmd = "gh auth token";
-            };
-            FIREFLY_TOKEN = {
-              hosts = ["firefly.m7.rs"];
-              cmd = "pass firefly.m7.rs/pi-pat";
-            };
-          };
-        };
-      };
     };
     keybindings = {
       "app.editor.external" = ["alt+e"];
     };
   };
   home.sessionVariables.PI_SKIP_VERSION_CHECK = true;
-
   home.file.".pi/agent/skills".source = ./skills;
-  home.file.".pi/agent/extensions".source = pkgs.buildNpmPackage {
-    pname = "pi-extensions";
-    version = "0-unstable";
-    src = ./extensions;
-    npmDepsHash = "sha256-odIhyMMSPBTtskV52CTEBGpMPVzUkBFLj6n+9JZCNZo=";
-    npmDepsFetcherVersion = 2;
-    dontNpmBuild = true;
-    installPhase = ''
-      runHook preInstall
-      mkdir -p $out
-      cp -r . $out/
-      runHook postInstall
-    '';
-  };
-  home.packages = [pkgs.qemu];
 }

home/gabriel/features/pi/extensions.nix

@@ -0,0 +1,38 @@
+{pkgs, ...}: {
+  programs.pi-coding-agent = {
+    settings = {
+      extensions = [
+        (pkgs.buildNpmPackage {
+          pname = "custom-extensions";
+          version = "0-unstable";
+          src = ./custom-extensions;
+          npmDepsHash = "sha256-odIhyMMSPBTtskV52CTEBGpMPVzUkBFLj6n+9JZCNZo=";
+          npmDepsFetcherVersion = 2;
+          dontNpmBuild = true;
+          installPhase = ''
+            runHook preInstall
+            mkdir -p $out
+            cp -r . $out/
+            runHook postInstall
+          '';
+        })
+      ];
+      gondolin = {
+        httpProxy = {
+          allowedHosts = ["api.github.com"  "firefly.m7.rs"];
+          secrets = {
+            GITHUB_TOKEN = {
+              hosts = ["api.github.com"];
+              cmd = "gh auth token";
+            };
+            FIREFLY_TOKEN = {
+              hosts = ["firefly.m7.rs"];
+              cmd = "pass firefly.m7.rs/pi-pat";
+            };
+          };
+        };
+      };
+    };
+  };
+  home.packages = [pkgs.qemu];
+}