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/clock/clock_test.go

@@ -0,0 +1,75 @@
+package clock
+
+import (
+	"context"
+	"encoding/json"
+	"strings"
+	"testing"
+	"time"
+
+	"gitea.stevedudenhoeffer.com/steve/majordomo/llm"
+)
+
+func fixed() time.Time {
+	return time.Date(2026, 6, 10, 15, 30, 0, 0, time.UTC)
+}
+
+func callTool(t *testing.T, name, args string) llm.ToolResult {
+	t.Helper()
+	sk := New(WithClock(fixed))
+	tool, ok := sk.Tools().Get(name)
+	if !ok {
+		t.Fatalf("tool %q missing", name)
+	}
+	return llm.ExecuteTool(context.Background(), tool, llm.ToolCall{
+		ID: "c1", Name: name, Arguments: json.RawMessage(args),
+	})
+}
+
+func TestTimeNowUTC(t *testing.T) {
+	res := callTool(t, "time_now", `{}`)
+	if res.IsError {
+		t.Fatalf("result = %+v", res)
+	}
+	var out map[string]string
+	if err := json.Unmarshal([]byte(res.Content), &out); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if out["rfc3339"] != "2026-06-10T15:30:00Z" || out["weekday"] != "Wednesday" {
+		t.Errorf("out = %v", out)
+	}
+}
+
+func TestTimeNowZoned(t *testing.T) {
+	res := callTool(t, "time_now", `{"timezone":"America/New_York"}`)
+	if res.IsError {
+		t.Fatalf("result = %+v", res)
+	}
+	if !strings.Contains(res.Content, "2026-06-10T11:30:00-04:00") {
+		t.Errorf("content = %s", res.Content)
+	}
+}
+
+func TestTimeNowBadZone(t *testing.T) {
+	res := callTool(t, "time_now", `{"timezone":"Mars/Olympus"}`)
+	if !res.IsError {
+		t.Errorf("result = %+v, want error", res)
+	}
+}
+
+func TestTimeConvert(t *testing.T) {
+	res := callTool(t, "time_convert", `{"time":"2026-06-10T15:30:00Z","timezone":"Europe/Berlin"}`)
+	if res.IsError {
+		t.Fatalf("result = %+v", res)
+	}
+	if !strings.Contains(res.Content, "2026-06-10T17:30:00+02:00") {
+		t.Errorf("content = %s", res.Content)
+	}
+}
+
+func TestInstructionsMentionTools(t *testing.T) {
+	sk := New()
+	if !strings.Contains(sk.Instructions(), "time_now") {
+		t.Errorf("instructions = %q", sk.Instructions())
+	}
+}