Add Sentry for error tracking

Add Sentry for error tracking

Optional sentry config in global config.fnl with SecretString DSN.
Best-effort init in main — logs a warning and continues if config is
missing, DSN resolution fails, or Sentry is not configured. Uses rustls
to avoid native OpenSSL dependency.

Config schema: {:sentry {:dsn \"https://key@sentry.io/123\"}}

Assisted-by: GLM-5.1 via pi