feat: add retrospective MCP tool

Adds internal/skills/retrospective/ — an MCP skill that reads a session
log and dispatches a worker subprocess (via ExecutorFn) to identify
learnings and write them to the brain. Follows the same executor pattern
as the TDD skill.

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

internal/skills/retrospective/handlers_test.go

@@ -0,0 +1,49 @@
+// internal/skills/retrospective/handlers_test.go
+package retrospective_test
+
+import (
+	"context"
+	"encoding/json"
+	"testing"
+
+	iexec "github.com/mathiasbq/supervisor/internal/exec"
+	"github.com/mathiasbq/supervisor/internal/skills/retrospective"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestHandle_Retrospective_RequiresSessionID(t *testing.T) {
+	s := retrospective.New(retrospective.Config{})
+	_, err := s.Handle(context.Background(), "retrospective", json.RawMessage(`{}`))
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "session_id")
+}
+
+func TestHandle_Retrospective_BuildsPromptWithSessionLog(t *testing.T) {
+	var capturedReq iexec.Request
+	s := retrospective.New(retrospective.Config{
+		SkillPrompt:  "retrospective discipline",
+		DefaultModel: "ollama/test",
+		SessionsDir:  t.TempDir(), // empty dir, no session file — that's OK, session.Read returns nil
+		ExecutorFn: func(_ context.Context, req iexec.Request) (iexec.Result, error) {
+			capturedReq = req
+			return iexec.Result{
+				Status:   "pass",
+				Phase:    "retrospective",
+				Skill:    "retrospective",
+				Verified: true,
+				Message:  "wrote 2 entries to brain",
+			}, nil
+		},
+	})
+
+	args, _ := json.Marshal(map[string]string{"session_id": "empty-session"})
+	out, err := s.Handle(context.Background(), "retrospective", args)
+	require.NoError(t, err)
+
+	var result iexec.Result
+	require.NoError(t, json.Unmarshal(out, &result))
+	assert.Equal(t, "pass", result.Status)
+	assert.Contains(t, capturedReq.SkillPrompt, "retrospective discipline")
+	assert.Contains(t, capturedReq.TaskPrompt, "empty-session")
+}