fix: add externalSkills option to pi agent module

Skills from flake inputs (e.g. Basecamp) were not being passed to
mkOpencodeSkills for the pi agent, so they never appeared in
~/.pi/agent/skills/. This adds the same externalSkills option that
the opencode agent module already has.

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

@@ -51,6 +51,44 @@ in {
       '';
     };
 
+    externalSkills = mkOption {
+      type = types.listOf (types.submodule {
+        options = {
+          src = mkOption {
+            type = types.anything;
+            description = "Flake input pointing to a skills repository root.";
+          };
+          skillsDir = mkOption {
+            type = types.str;
+            default = "skills";
+            description = ''
+              Subdirectory inside src that contains skill folders.
+            '';
+          };
+          selectSkills = mkOption {
+            type = types.nullOr (types.listOf types.str);
+            default = null;
+            description = ''
+              List of skill names to cherry-pick from this source.
+              null means include every skill found in skillsDir.
+            '';
+          };
+        };
+      });
+      default = [];
+      description = ''
+        External skill sources passed to mkOpencodeSkills.
+        Each entry maps directly to an element of the externalSkills
+        list accepted by the AGENTS flake's lib.mkOpencodeSkills.
+      '';
+      example = literalExpression ''
+        [
+          { src = inputs.skills-anthropic; selectSkills = [ "claude-api" ]; }
+          { src = inputs.basecamp; }
+        ]
+      '';
+    };
+
     settings = mkOption {
       type = types.submodule {
         freeformType = types.attrsOf types.anything;
@@ -222,11 +260,18 @@ in {
       # ── Agents — pi-subagents .md files ────────────────────────────
       agentFiles
 
-      # ── Skills symlinked from AGENTS repo ──────────────────────────
+      # ── Skills symlinked from AGENTS repo + external skills ────────
       (mkIf (cfg.agentsInput != null) {
         ".pi/agent/skills".source = cfg.agentsInput.lib.mkOpencodeSkills {
           inherit pkgs;
           customSkills = "${cfg.agentsInput}/skills";
+          externalSkills =
+            map (
+              entry:
+                {inherit (entry) src skillsDir;}
+                // optionalAttrs (entry.selectSkills != null) {inherit (entry) selectSkills;}
+            )
+            cfg.externalSkills;
         };
       })
     ];