posthog-upgrade-migration

Upgrade posthog-js and posthog-node SDKs with breaking change detection. Covers v4 to v5 posthog-node migration (sendFeatureFlags change), posthog-js autocapture API changes, and version-specific gotchas. Trigger: "upgrade posthog", "posthog breaking changes", "update posthog SDK", "posthog version", "posthog migration".