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/config/config.go

@@ -206,8 +206,9 @@ type QuotaExceeded struct {
 	// SwitchPreviewModel indicates whether to automatically switch to a preview model when a quota is exceeded.
 	SwitchPreviewModel bool `yaml:"switch-preview-model" json:"switch-preview-model"`
 
-	// AntigravityCredits indicates whether to retry Antigravity quota_exhausted 429s once
-	// on the same credential with enabledCreditTypes=["GOOGLE_ONE_AI"].
+	// AntigravityCredits enables credits-based last-resort fallback for Claude models.
+	// When all free-tier auths are exhausted (429/503), the conductor retries with
+	// an auth that has available Google One AI credits.
 	AntigravityCredits bool `yaml:"antigravity-credits" json:"antigravity-credits"`
 }