feat(runtime): add pi-coding-agent package with CLI tools

- Added pi-coding-agent package definition with buildNpmPackage
- Includes npm dependencies and native/build inputs for GUI libraries
- Skips generate-models step during build (models already in repo)
- Creates 'pi' executable pointing to coding-agent dist/cli.js

Fixes build issues where generate-models would fail during build phase

packages/pi-coding-agent/default.nix

@@ -0,0 +1,85 @@
+{ lib
+, buildNpmPackage
+, fetchFromGitHub
+, nodejs
+, pkg-config
+, pixman
+, cairo
+, pango
+, libjpeg
+, giflib
+, librsvg
+,
+}:
+
+buildNpmPackage rec {
+  pname = "pi-coding-agent";
+  version = "0.51.6";
+
+  src = fetchFromGitHub {
+    owner = "badlogic";
+    repo = "pi-mono";
+    rev = "v${version}";
+    hash = "sha256-P9IXfHr3brqCP8eQVLn1Sl6yJ9Bx7weqxyk6iHKWZpo=";
+  };
+
+  npmDepsHash = "sha256-dDh9nk3w1pevSbOV20pFAqmHGcjTw9HUV0Z1BLYMNJU=";
+
+  nativeBuildInputs = [ pkg-config ];
+
+  buildInputs = [
+    pixman
+    cairo
+    pango
+    libjpeg
+    giflib
+    librsvg
+  ];
+
+  # Skip generate-models in ai package (models.generated.ts already in repo)
+  preBuild = ''
+    substituteInPlace packages/ai/package.json \
+      --replace-fail '"build": "npm run generate-models && tsgo -p tsconfig.build.json"' \
+                     '"build": "tsgo -p tsconfig.build.json"'
+  '';
+
+  # Build coding-agent dependencies in order
+  buildPhase = ''
+    runHook preBuild
+
+    cd packages/tui && npm run build && cd ../..
+    cd packages/ai && npm run build && cd ../..
+    cd packages/agent && npm run build && cd ../..
+    cd packages/coding-agent && npm run build && cd ../..
+
+    runHook postBuild
+  '';
+
+  installPhase = ''
+      runHook preInstall
+
+      mkdir -p $out/lib/pi-coding-agent $out/bin
+
+      # Copy node_modules and the packages directory
+      cp -r node_modules $out/lib/pi-coding-agent/
+      cp -r packages $out/lib/pi-coding-agent/
+
+      cat > $out/bin/pi <<EOF
+    #!${nodejs}/bin/node
+    import('$out/lib/pi-coding-agent/packages/coding-agent/dist/cli.js');
+    EOF
+      chmod +x $out/bin/pi
+
+      runHook postInstall
+  '';
+
+  meta = {
+    description = "Coding agent CLI with read, bash, edit, write tools and session management";
+    homepage = "https://github.com/badlogic/pi-mono";
+    downloadPage = "https://www.npmjs.com/package/@mariozechner/pi-coding-agent";
+    changelog = "https://github.com/badlogic/pi-mono/blob/main/packages/coding-agent/CHANGELOG.md";
+    license = lib.licenses.mit;
+    maintainers = with lib.maintainers; [ evanreichard ];
+    mainProgram = "pi";
+  };
+}