pi-agent stack (#46)

* pi-agent stack

* fix: hardcode pi binary path, set XDG env for npm

npm prefix discovery fails in systemd and activation contexts
because NPM_CONFIG_USERCONFIG and XDG_DATA_HOME are not set.
Hardcode the known path ~/.local/share/npm/bin/pi instead.

home/common.nix

@@ -21,6 +21,7 @@
     ./mise.nix
     ./migration.nix
     ./nvim.nix
+    ./pi.nix
     ./prompt.nix
     ./skills.nix
     ./scripts.nix

home/pi.nix

@@ -0,0 +1,52 @@
+{
+  config,
+  lib,
+  pkgs,
+  hostConfig,
+  ...
+}:
+let
+  npmDir = "${config.xdg.dataHome}/npm";
+  piBin = "${npmDir}/bin/pi";
+in
+lib.mkIf hostConfig.isLinux {
+  # Install pi-coding-agent globally via npm at activation time.
+  home.activation.installPiAgent = lib.hm.dag.entryAfter [ "writeBoundary" ] ''
+    export PATH="${
+      lib.makeBinPath [
+        pkgs.nodejs_22
+        pkgs.coreutils
+      ]
+    }:$PATH"
+    export NPM_CONFIG_USERCONFIG="${config.xdg.configHome}/npm/npmrc"
+    export XDG_DATA_HOME="${config.xdg.dataHome}"
+    export XDG_CACHE_HOME="${config.xdg.cacheHome}"
+
+    if [ ! -d "${npmDir}/lib/node_modules/@mariozechner/pi-coding-agent" ]; then
+      npm install -g @mariozechner/pi-coding-agent 2>/dev/null || true
+    fi
+  '';
+
+  # Install Pi extensions at activation time:
+  #   - @e9n/pi-channels: Telegram/Slack bridge with RPC-based persistent sessions
+  #   - pi-schedule-prompt: cron/interval scheduled prompts
+  #   - pi-subagents: background task delegation with async execution
+  home.activation.installPiExtensions = lib.hm.dag.entryAfter [ "installPiAgent" ] ''
+    export PATH="${
+      lib.makeBinPath [
+        pkgs.nodejs_22
+        pkgs.coreutils
+        pkgs.git
+      ]
+    }:$PATH"
+    export NPM_CONFIG_USERCONFIG="${config.xdg.configHome}/npm/npmrc"
+    export XDG_DATA_HOME="${config.xdg.dataHome}"
+    export XDG_CACHE_HOME="${config.xdg.cacheHome}"
+
+    if [ -x "${piBin}" ]; then
+      for pkg in "@e9n/pi-channels" "pi-schedule-prompt" "pi-subagents"; do
+        "${piBin}" install "npm:$pkg" 2>/dev/null || true
+      done
+    fi
+  '';
+}