feat(runtime-fallback): generalize provider auto-retry signal detection

Refactor retry signal detection to be provider-agnostic:
- Replace hardcoded Copilot/OpenAI checks with generic pattern matching
- Detect any provider message containing limit/quota keywords + [retrying in X]
- Add OpenAI pattern: 'usage limit has been reached [retrying in X]'
- Update logging to use generic 'provider' instead of specific names
- Add 'usage limit has been reached' to RETRYABLE_ERROR_PATTERNS

This enables fallback escalation for any provider that signals automatic
retries due to quota/rate limits, not just Copilot and OpenAI.

Closes PR discussion: generalize retry pattern detection

src/hooks/runtime-fallback/constants.ts

@@ -26,6 +26,7 @@ export const RETRYABLE_ERROR_PATTERNS = [
   /rate.?limit/i,
   /too.?many.?requests/i,
   /quota.?exceeded/i,
+  /usage\s+limit\s+has\s+been\s+reached/i,
   /service.?unavailable/i,
   /overloaded/i,
   /temporarily.?unavailable/i,