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/exec/result.go

@@ -20,7 +20,12 @@ type Result struct {
 }
 
 var validStatuses = map[string]bool{"pass": true, "fail": true, "error": true}
-var validPhases = map[string]bool{"red": true, "green": true, "refactor": true}
+var validPhases = map[string]bool{
+	"red":           true,
+	"green":         true,
+	"refactor":      true,
+	"retrospective": true,
+}
 
 func (r Result) Validate() error {
 	var errs []string