feat(pi): add guardrails config option for pi-guardrails integration

Adds a guardrails submodule option to coding.agents.pi that:
- Generates ~/.pi/agent/extensions/guardrails.json when enabled
- Automatically injects @aliou/pi-guardrails package into settings.packages
- Provides structured options for policies, pathAccess, and permissionGate

The module generates the JSON config that pi-guardrails reads for
its security hooks (policies, permission-gate, path-access).

Limitations documented in option descriptions:
- Path access checks are lexical (not symlink-safe)
- Local project guardrails.json can override global rule IDs

modules/home-manager/coding/agents/pi.nix

@@ -202,6 +202,37 @@
           See pi docs/settings.md for all options.
         '';
       };
+
+      # ── Pi Guardrails ─────────────────────────────────────────────
+      guardrails = mkOption {
+        type = types.nullOr (types.submodule {
+          options = {
+            enable = mkEnableOption
+              "Generate ~/.pi/agent/extensions/guardrails.json for pi-guardrails. "
+              + "Adds @aliou/pi-guardrails to packages automatically.";
+
+            config = mkOption {
+              type = types.attrsOf types.anything;
+              default = {};
+              description = ''
+                Guardrails configuration written to ~/.pi/agent/extensions/guardrails.json.
+                See https://github.com/aliou/pi-guardrails for config schema.
+
+                IMPORTANT: Path access checks are lexical (not symlink-safe).
+                Local project .pi/extensions/guardrails.json can override same rule IDs
+                (memory > local > global > defaults). For immutable global policies,
+                consider a wrapper or upstream patch.
+              '';
+            };
+          };
+        });
+        default = null;
+        description = ''
+          Pi Guardrails security configuration.
+          Generates ~/.pi/agent/extensions/guardrails.json when enabled.
+          The @aliou/pi-guardrails package is added to settings.packages automatically.
+        '';
+      };
     };
 
   config = with lib; let
@@ -224,8 +255,33 @@
           attrs
         );
 
+      # Base settings (already filtered)
       piSettings = filterNulls cfg.settings;
 
+      # Guardrails package to inject when guardrails is enabled
+      guardrailsPackage = "npm:@aliou/pi-guardrails@0.11.1";
+
+      # Guardrails config (only when guardrails is enabled)
+      guardrailsJson = if (cfg.guardrails != null && cfg.guardrails.enable)
+        then builtins.toJSON cfg.guardrails.config
+        else null;
+
+      # Merge guardrails package into settings.packages when guardrails is enabled
+      piSettingsWithGuardrails = let
+        baseSettings = cfg.settings;
+        basePackages = baseSettings.packages or [];
+        hasGuardrailsPackage = lib.any
+          (p: lib.hasPrefix "npm:@aliou/pi-guardrails" p ||
+            (lib.hasPrefix "git:" p && lib.hasSuffix "/pi-guardrails" p))
+          basePackages;
+        packagesWithGuardrails = if (cfg.guardrails != null && cfg.guardrails.enable && !hasGuardrailsPackage)
+          then basePackages ++ [guardrailsPackage]
+          else basePackages;
+      in
+        if packagesWithGuardrails != basePackages
+        then filterNulls (baseSettings // {packages = packagesWithGuardrails;})
+        else piSettings;
+
       # Coding rules config for renderForPi (only when both agentsInput and codingRules are set)
       piCodingRules =
         if cfg.agentsInput != null && cfg.codingRules != null
@@ -269,10 +325,16 @@
 
         # ── ~/.pi/agent/settings.json ──────────────────────────────────
         {
-          ".pi/agent/settings.json".text = builtins.toJSON piSettings;
+          ".pi/agent/settings.json".text = builtins.toJSON piSettingsWithGuardrails;
           ".pi/agent/settings.json".force = true;
         }
 
+        # ── pi-guardrails config ─────────────────────────────────────
+        (mkIf (guardrailsJson != null) {
+          ".pi/agent/extensions/guardrails.json".text = guardrailsJson;
+          ".pi/agent/extensions/guardrails.json".force = true;
+        })
+
         # ── AGENTS.md — agent descriptions and specialist listing ──────
         (mkIf (cfg.agentsInput != null) {
           ".pi/agent/AGENTS.md".source = "${rendered}/AGENTS.md";