feat(pi): manage global subagent configuration

AGENTS.md

@@ -83,3 +83,8 @@ The domain model (Host, Module, Skeleton, Auto-loader, Enable convention, overla
 - Pi's tool discovery checks `~/.pi/agent/bin` before `PATH`, and downloaded generic Linux binaries there can be unusable on NixOS with the stub-ld error.
   This flake patches Pi to validate local tool binaries before selecting them, so it falls back to usable `fd`/`rg` from `PATH` instead.
   Stale unpatched launchers are the remaining failure mode for broken `@` autocomplete.
+- Nix flake evaluation ignores untracked files in this checkout.
+  Keep a new auto-loaded module staged or committed until it is removed, otherwise `nix flake check` and `nixos-rebuild --flake` evaluate without it and report its options as missing.
+- The current Steam desktop client is an XWayland application.
+  Its CEF windows do not support Ozone and Steam composites them into an SDL surface with X11 extensions, so SDL Wayland selectors do not make the visible client native Wayland.
+  Keep fractional scaling sharp with Hyprland's `xwayland.force_zero_scaling` and Steam's own `STEAM_FORCE_DESKTOPUI_SCALING` instead.

hosts/neogaia/default.nix

@@ -68,9 +68,11 @@
   modules.agents.herdr.enable = true;
   modules.agents.tools.gitea-axi.enable = true;
   modules.agents.pi.enable = true;
+  modules.agents.pi.subagents.maxConcurrent = 8;
 
   modules.desktop.enable = true;
   modules.desktop.obsidian.enable = true;
+  modules.desktop.steam.enable = true;
 
   time.timeZone = "America/New_York";
   i18n.defaultLocale = "en_GB.UTF-8";

modules/agents/pi/pi.nix

@@ -11,6 +11,40 @@ let
   cfg = config.modules.agents.pi;
   user = config.user.name;
   piDir = "${config.users.users.${user}.home}/.pi/agent";
+  reservedToolProfiles = [
+    "none"
+    "read-only"
+    "read-only-with-safe-bash"
+    "full-tools"
+  ];
+  subagentsConfig =
+    lib.optionalAttrs (cfg.subagents.defaultContext != null) {
+      defaultContext = cfg.subagents.defaultContext;
+    }
+    // lib.optionalAttrs (cfg.subagents.defaultTools != null) {
+      defaultTools = cfg.subagents.defaultTools;
+    }
+    // lib.optionalAttrs (cfg.subagents.maxConcurrent != null) {
+      maxConcurrent = cfg.subagents.maxConcurrent;
+    }
+    // lib.optionalAttrs (cfg.subagents.recentTerminalTtlMs != null) {
+      recentTerminalTtlMs = cfg.subagents.recentTerminalTtlMs;
+    }
+    // lib.optionalAttrs (
+      cfg.subagents.ui.enabled != null || cfg.subagents.ui.defaultExpanded != null
+    ) {
+      ui =
+        lib.optionalAttrs (cfg.subagents.ui.enabled != null) {
+          enabled = cfg.subagents.ui.enabled;
+        }
+        // lib.optionalAttrs (cfg.subagents.ui.defaultExpanded != null) {
+          defaultExpanded = cfg.subagents.ui.defaultExpanded;
+        };
+    }
+    // lib.optionalAttrs (cfg.subagents.toolProfiles != { }) {
+      toolProfiles = cfg.subagents.toolProfiles;
+    };
+  subagentsJson = (pkgs.formats.json { }).generate "pi-subagents.json" subagentsConfig;
   patchedPi = pkgs.pi-coding-agent.overrideAttrs (old: {
     patches = (old.patches or [ ]) ++ [
       ./patches/pi-flex-spacer.patch
@@ -39,10 +73,112 @@ let
   };
 in
 {
-  options.modules.agents.pi.enable = lib.mkEnableOption ''
-    Pi, a terminal coding agent, configured via home-manager'';
+  options.modules.agents.pi = {
+    enable = lib.mkEnableOption ''
+      Pi, a terminal coding agent, configured via home-manager'';
+
+    subagents = {
+      defaultContext = lib.mkOption {
+        type = lib.types.nullOr (lib.types.enum [
+          "independent"
+          "fork"
+        ]);
+        default = null;
+        description = ''
+          Default context mode for subagents.
+          Left null, the extension keeps its in-code default.
+        '';
+      };
+
+      defaultTools = lib.mkOption {
+        type = lib.types.nullOr lib.types.str;
+        default = null;
+        example = "read-only-with-safe-bash";
+        description = ''
+          Default tool profile for subagents.
+          Left null, the extension keeps its in-code default.
+        '';
+      };
+
+      maxConcurrent = lib.mkOption {
+        type = lib.types.nullOr lib.types.ints.positive;
+        default = null;
+        example = 4;
+        description = ''
+          Maximum number of child processes allowed to run concurrently.
+          Left null, the extension keeps its in-code default.
+        '';
+      };
+
+      recentTerminalTtlMs = lib.mkOption {
+        type = lib.types.nullOr lib.types.ints.unsigned;
+        default = null;
+        example = 600000;
+        description = ''
+          Milliseconds to retain terminal subagents in the recent work set.
+          Zero disables time-based retention.
+          Left null, the extension keeps its in-code default.
+        '';
+      };
+
+      ui = {
+        enabled = lib.mkOption {
+          type = lib.types.nullOr lib.types.bool;
+          default = null;
+          description = ''
+            Whether the extension renders its built-in subagent monitor.
+            Left null, the extension keeps its in-code default.
+          '';
+        };
+
+        defaultExpanded = lib.mkOption {
+          type = lib.types.nullOr lib.types.bool;
+          default = null;
+          description = ''
+            Whether the built-in subagent monitor starts expanded.
+            Left null, the extension keeps its in-code default.
+          '';
+        };
+      };
+
+      toolProfiles = lib.mkOption {
+        type = lib.types.attrsOf (
+          lib.types.submodule {
+            options.activeTools = lib.mkOption {
+              type = lib.types.listOf lib.types.str;
+              description = "Pi tools made available to a child using this profile.";
+            };
+          }
+        );
+        default = { };
+        example = {
+          review = {
+            activeTools = [
+              "read"
+              "grep"
+              "find"
+              "ls"
+            ];
+          };
+        };
+        description = ''
+          Custom named tool profiles for subagents.
+          The extension's reserved built-in profile names cannot be redefined.
+        '';
+      };
+    };
+  };
 
   config = lib.mkIf cfg.enable {
+    assertions = [
+      {
+        assertion = lib.intersectLists reservedToolProfiles (
+          builtins.attrNames cfg.subagents.toolProfiles
+        ) == [ ];
+        message = "modules.agents.pi.subagents.toolProfiles may not redefine the reserved profiles: ${lib.concatStringsSep ", " reservedToolProfiles}.";
+      }
+    ];
+
     home-manager.users.${user} = {
       programs.pi-coding-agent = {
         enable = true;
@@ -58,21 +194,29 @@ in
         };
       };
 
-      home.file = {
-        # The first declarative rollout replaces the interactive settings file.
-        # Login state stays in auth.json, which this module does not manage.
-        "${piDir}/settings.json".force = true;
+      home.file =
+        {
+          # The first declarative rollout replaces the interactive settings file.
+          # Login state stays in auth.json, which this module does not manage.
+          "${piDir}/settings.json".force = true;
 
-        "${piDir}/extensions" = {
-          source = piExtensions;
-          recursive = true;
-        };
+          "${piDir}/extensions" = {
+            source = piExtensions;
+            recursive = true;
+          };
 
-        "${piDir}/prompts" = {
-          source = ./prompts;
-          recursive = true;
+          "${piDir}/prompts" = {
+            source = ./prompts;
+            recursive = true;
+          };
+        }
+        // lib.optionalAttrs (subagentsConfig != { }) {
+          # Declaring any global override makes Nix the owner of the runtime file.
+          "${piDir}/subagents.json" = {
+            source = subagentsJson;
+            force = true;
+          };
         };
-      };
     };
   };
 }

modules/desktop/hyprland/hyprland.nix

@@ -112,6 +112,10 @@ in
           blur.enabled = cfg.blur;
         };
 
+        # XWayland clients render at the panel's native resolution instead of
+        # being raster-scaled by the compositor at the fractional monitor scale.
+        xwayland.force_zero_scaling = true;
+
         animations = {
           enabled = true;
           bezier = [ "ease, 0.25, 0.1, 0.25, 1.0" ];

modules/desktop/steam.nix

@@ -0,0 +1,24 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# Steam game launcher and runtime integration.
+let
+  cfg = config.modules.desktop.steam;
+in
+{
+  options.modules.desktop.steam.enable = lib.mkEnableOption "Steam game launcher";
+
+  config = lib.mkIf cfg.enable {
+    programs.steam = {
+      enable = true;
+      package = pkgs.steam.override {
+        extraEnv.STEAM_FORCE_DESKTOPUI_SCALING = "1.5";
+      };
+    };
+
+    hardware.steam-hardware.enable = true;
+  };
+}

modules/desktop/waybar.nix

@@ -83,6 +83,7 @@ in
             "class<chromium.*>" = g "f268";
             "class<[Cc]ode>" = g "f121";
             "class<obsidian>" = g "f02d";
+            "class<[Ss]team>" = g "f1b6";
           };
         };