fix(posthog): disable exception autocapture to stay within free tier

Error tracking exceeded 100K free tier limit (188K in 5 days).

Top exceptions were mostly noise:
- ProviderModelNotFoundError: 75K (user config issues)
- EPIPE/EOF/stream destroyed: 40K (normal pipe closures)
- ENOSPC: 3K (user disk space issues)

Manual captureException() for critical errors in runner.ts
is preserved. Only automatic unhandled exception capture is
disabled.