fix: extend valid phases and return empty slice for missing session

Add "retrospective" to validPhases so non-TDD skills pass Validate().
Return []Entry{} instead of nil in session.Read when no file exists,
so JSON serialisation produces [] rather than null.

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

internal/session/session.go

@@ -68,7 +68,7 @@ func Read(sessionsDir, sessionID string) ([]Entry, error) {
 	path := filepath.Join(sessionsDir, sessionID+".jsonl")
 	f, err := os.Open(path)
 	if errors.Is(err, fs.ErrNotExist) {
-		return nil, nil
+		return []Entry{}, nil
 	}
 	if err != nil {
 		return nil, fmt.Errorf("open session log: %w", err)