feat(spec): add spec writing MCP skill

Adds the spec skill that generates structured implementation specs from
requirements and writes them to a configurable output path in the project.
Follows the same pattern as review/debug skills with session history injection.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/skills/spec/handlers_test.go

@@ -0,0 +1,61 @@
+// internal/skills/spec/handlers_test.go
+package spec_test
+
+import (
+	"context"
+	"encoding/json"
+	"testing"
+
+	iexec "github.com/mathiasbq/supervisor/internal/exec"
+	"github.com/mathiasbq/supervisor/internal/skills/spec"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSpecToolRegistered(t *testing.T) {
+	sk := spec.New(spec.Config{SkillPrompt: "spec rules"})
+	names := make([]string, 0)
+	for _, tool := range sk.Tools() {
+		names = append(names, tool.Name)
+	}
+	assert.Contains(t, names, "spec")
+}
+
+func TestSpecRequiresProjectRoot(t *testing.T) {
+	sk := spec.New(spec.Config{SkillPrompt: "s"})
+	_, err := sk.Handle(context.Background(), "spec", json.RawMessage(`{"requirements":"add login"}`))
+	assert.ErrorContains(t, err, "project_root")
+}
+
+func TestSpecRequiresRequirements(t *testing.T) {
+	sk := spec.New(spec.Config{SkillPrompt: "s"})
+	_, err := sk.Handle(context.Background(), "spec", json.RawMessage(`{"project_root":"/tmp"}`))
+	assert.ErrorContains(t, err, "requirements")
+}
+
+func TestSpecCallsExecutor(t *testing.T) {
+	called := false
+	var capturedTask string
+	fakeFn := func(_ context.Context, req iexec.Request) (iexec.Result, error) {
+		called = true
+		capturedTask = req.TaskPrompt
+		return iexec.Result{
+			Status: "pass", Phase: "spec", Skill: "spec",
+			FilePath: "/tmp/proj/docs/login-spec.md",
+			Verified: true, ModelUsed: "self", Message: "spec written: login feature",
+		}, nil
+	}
+
+	sk := spec.New(spec.Config{SkillPrompt: "spec rules", ExecutorFn: fakeFn, SessionsDir: t.TempDir()})
+	out, err := sk.Handle(context.Background(), "spec", json.RawMessage(
+		`{"project_root":"/tmp/proj","requirements":"add OAuth2 login","output_path":"docs/login-spec.md"}`,
+	))
+	require.NoError(t, err)
+	assert.True(t, called)
+	assert.Contains(t, capturedTask, "OAuth2 login")
+	assert.Contains(t, capturedTask, "docs/login-spec.md")
+
+	var result iexec.Result
+	require.NoError(t, json.Unmarshal(out, &result))
+	assert.Equal(t, "spec", result.Phase)
+}