feat: skills — additive instruction+tool bundles, clock + calc examples

Phase 6: skill.New constructor satisfying the agent.Skill contract;
instruction-only skills; ordered additive composition; skill/clock
(injectable-clock time tools) and skill/calc (recursive-descent arithmetic
evaluator) as ready-made examples with full test suites incl. an
agent-loop round trip. ADR-0013; README skills section + matrix synced.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

skill/calc/calc_test.go

@@ -0,0 +1,78 @@
+package calc
+
+import (
+	"context"
+	"encoding/json"
+	"strings"
+	"testing"
+
+	"gitea.stevedudenhoeffer.com/steve/majordomo/agent"
+	"gitea.stevedudenhoeffer.com/steve/majordomo/llm"
+	"gitea.stevedudenhoeffer.com/steve/majordomo/provider/fake"
+)
+
+func TestEval(t *testing.T) {
+	tests := []struct {
+		expr string
+		want float64
+	}{
+		{"1+2", 3},
+		{"2 * 3 + 4", 10},
+		{"2 + 3 * 4", 14},
+		{"(2 + 3) * 4", 20},
+		{"10 / 4", 2.5},
+		{"-3 + 5", 2},
+		{"--3", 3},
+		{"2^10", 1024},
+		{"2^3^2", 512}, // right-associative
+		{"-2^2", -4},   // unary binds looser than power
+		{"7 % 3", 1},
+		{"1.5e2 + 1", 151},
+		{" ( 1 + 1 ) * ( 2 + 2 ) ", 8},
+	}
+	for _, tt := range tests {
+		got, err := Eval(tt.expr)
+		if err != nil {
+			t.Errorf("Eval(%q): %v", tt.expr, err)
+			continue
+		}
+		if got != tt.want {
+			t.Errorf("Eval(%q) = %v, want %v", tt.expr, got, tt.want)
+		}
+	}
+}
+
+func TestEvalErrors(t *testing.T) {
+	for _, expr := range []string{"", "1/0", "5 % 0", "2 +", "(1+2", "1 + abc", "1 2", "2^99999"} {
+		if _, err := Eval(expr); err == nil {
+			t.Errorf("Eval(%q) should error", expr)
+		}
+	}
+}
+
+// TestSkillInAgentLoop: an agent actually invokes calculate and answers
+// from its result.
+func TestSkillInAgentLoop(t *testing.T) {
+	fp := fake.New("fp")
+	fp.Enqueue("m",
+		fake.ReplyWith(llm.Response{
+			ToolCalls:    []llm.ToolCall{{ID: "c1", Name: "calculate", Arguments: json.RawMessage(`{"expression":"(2+3)*4"}`)}},
+			FinishReason: llm.FinishToolCalls,
+		}),
+		fake.Reply("the answer is 20"),
+	)
+	m, _ := fp.Model("m")
+
+	a := agent.New(m, "Math helper.", agent.WithSkill(New()))
+	res, err := a.Run(context.Background(), "what is (2+3)*4?")
+	if err != nil {
+		t.Fatalf("Run: %v", err)
+	}
+	if res.Output != "the answer is 20" {
+		t.Errorf("output = %q", res.Output)
+	}
+	result := res.Steps[0].Results[0]
+	if result.IsError || !strings.Contains(result.Content, `"result":20`) {
+		t.Errorf("tool result = %+v", result)
+	}
+}