C0: address verified gadfly findings (trivial fixes)

From the PR #8 review (all graded in the gadfly MCP):
- skip empty palette names + dedupe by final tool name, instead of producing a
  "skill__" tool or an opaque box.Add duplicate error.
- delegationResult: no trailing blank line when a non-ok child produced no output.
- delegationErr: fold a child's partial output into the hard-failure error so it
  isn't silently dropped.

Deferred to C0b (design-level, not trivial): route delegation through the
tool.Registry gate/audit wrappers; expose the skill's real input schema to the
LLM instead of a generic inputs map. typed-nil PaletteSource is left as a caller
contract (the == nil guard catches the untyped-nil interface).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

run/palette_test.go

@@ -99,3 +99,27 @@ func TestNoPaletteNoDelegationTools(t *testing.T) {
 		t.Fatalf("nil-palette run failed: %v / %q", res.Err, res.Output)
 	}
 }
+
+// TestDelegationDedupeAndEmptySkip: empty + duplicate palette names are skipped,
+// not turned into "skill__"/duplicate tools that error at box.Add (gadfly C0).
+func TestDelegationDedupeAndEmptySkip(t *testing.T) {
+	pal := &recordingPalette{}
+	fp := fake.New("fake")
+	fp.Enqueue("m", fake.Reply("ok"))
+	m, _ := fp.Model("m")
+	ex := run.New(run.Config{
+		Registry: tool.NewRegistry(),
+		Models:   func(ctx context.Context, _ string) (context.Context, llm.Model, error) { return ctx, m, nil },
+		Ports:    run.Ports{Palette: pal},
+	})
+	// "" (empty) and a duplicate "helper" must not break the build.
+	res := ex.Run(context.Background(),
+		run.RunnableAgent{Name: "x", ModelTier: "m", SkillPalette: []string{"helper", "", "helper"}},
+		tool.Invocation{RunID: "r"}, "hi")
+	if res.Err != nil {
+		t.Fatalf("empty/duplicate palette names should be skipped, not error: %v", res.Err)
+	}
+	if res.Output != "ok" {
+		t.Fatalf("output = %q", res.Output)
+	}
+}