feat: debug-log the capability-gate verdict per add + SupportState.String()

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6

controller/internal/agentapi/features.go

@@ -35,6 +35,18 @@ const (
 	SupportNo
 )
 
+// String returns the state's wire/template vocabulary: "yes" | "no" | "unknown".
+func (s SupportState) String() string {
+	switch s {
+	case SupportYes:
+		return "yes"
+	case SupportNo:
+		return "no"
+	default:
+		return "unknown"
+	}
+}
+
 // SupportProber is the minimal agent surface a probe needs. *Client satisfies it, and so does the
 // web layer's netAgent seam — tests inject fakes there.
 type SupportProber interface {