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.go

@@ -0,0 +1,70 @@
+// internal/skills/retrospective/handlers.go
+package retrospective
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	iexec "github.com/mathiasbq/supervisor/internal/exec"
+	"github.com/mathiasbq/supervisor/internal/session"
+)
+
+type retroArgs struct {
+	SessionID string `json:"session_id"`
+	Model     string `json:"model,omitempty"`
+}
+
+// Handle dispatches the retrospective tool call.
+func (s *Skill) Handle(ctx context.Context, tool string, args json.RawMessage) (json.RawMessage, error) {
+	if tool != "retrospective" {
+		return nil, fmt.Errorf("unknown retrospective tool: %s", tool)
+	}
+	var a retroArgs
+	if err := json.Unmarshal(args, &a); err != nil {
+		return nil, fmt.Errorf("parse args: %w", err)
+	}
+	if a.SessionID == "" {
+		return nil, fmt.Errorf("session_id is required")
+	}
+
+	model := a.Model
+	if model == "" {
+		model = s.cfg.DefaultModel
+	}
+
+	// Read session log entries (empty slice if no log exists yet).
+	entries, err := session.Read(s.cfg.SessionsDir, a.SessionID)
+	if err != nil {
+		return nil, fmt.Errorf("read session log: %w", err)
+	}
+
+	logJSON, err := json.MarshalIndent(entries, "", "  ")
+	if err != nil {
+		return nil, fmt.Errorf("marshal session log: %w", err)
+	}
+
+	taskPrompt := fmt.Sprintf(
+		"SESSION_ID: %s\n\nSESSION_LOG:\n%s\n\nReview this session log. Identify what is novel or worth preserving as organizational knowledge. Write structured entries to brain/raw/ via brain_write. Return JSON result when done.",
+		a.SessionID, string(logJSON),
+	)
+
+	if s.cfg.ExecutorFn == nil {
+		return nil, fmt.Errorf("no executor configured")
+	}
+	result, err := s.cfg.ExecutorFn(ctx, iexec.Request{
+		SkillPrompt: s.cfg.SkillPrompt,
+		TaskPrompt:  taskPrompt,
+		Model:       model,
+		Tools:       "Bash,Read,Write",
+	})
+	if err != nil {
+		return nil, fmt.Errorf("retrospective worker: %w", err)
+	}
+
+	b, err := json.Marshal(result)
+	if err != nil {
+		return nil, fmt.Errorf("marshal result: %w", err)
+	}
+	return b, nil
+}