feat(antigravity): conductor-level credits fallback for Claude models

Move credits handling from executor-level retry to conductor-level
orchestration. When all free-tier auths are exhausted (429/503), the
conductor discovers auths with available Google One AI credits and
retries with enabledCreditTypes injected via context flag.

Key changes:
- Add AntigravityCreditsHint system for tracking per-auth credits state
- Conductor tries credits fallback after all auths fail (Execute/Stream/Count)
- Executor injects enabledCreditTypes only when conductor sets context flag
- Credits fallback respects provider scope (requires antigravity in providers)
- Add context cancellation check in credits fallback to avoid wasted requests
- Remove executor-level attemptCreditsFallback and preferCredits machinery
- Restructure 429 decision logic (parse details first, keyword fallback)
- Expand shouldAbort to cover INVALID_ARGUMENT/FAILED_PRECONDITION/500+UNKNOWN
- Support human-readable retry delay parsing (e.g. "1h43m56s")

internal/runtime/executor/gemini_cli_executor.go

@@ -898,7 +898,14 @@ func parseRetryDelay(errorBody []byte) (*time.Duration, error) {
 		if matches := re.FindStringSubmatch(message); len(matches) > 1 {
 			seconds, err := strconv.Atoi(matches[1])
 			if err == nil {
-				return new(time.Duration(seconds) * time.Second), nil
+				duration := time.Duration(seconds) * time.Second
+				return &duration, nil
+			}
+		}
+		reHuman := regexp.MustCompile(`after\s+((?:\d+h)?(?:\d+m)?(?:\d+s)?)\.?`)
+		if matches := reHuman.FindStringSubmatch(strings.ToLower(message)); len(matches) > 1 {
+			if duration, err := time.ParseDuration(matches[1]); err == nil && duration > 0 {
+				return &duration, nil
 			}
 		}
 	}