feature-flags

Wire PostHog feature flags end-to-end — server-side evaluation, client-side hook, and local dev overrides. Reads the project first, extends the existing PostHog setup if present.

## Two things that silently break PostHog flags

1. **Evaluating flags client-side when you need server-side behavior.** `useFeatureFlagEnabled` runs in the browser after hydration. If you're using a flag to show/hide a page section, that section flashes on then disappears. For anything above the fold or that affects routing, evaluate the flag on the server.
2. **Missing `distinctId` in server-side calls.** PostHog's server SDK requires a distinct user identifier to evaluate flags. Using a placeholder like `"anonymous"` means every unauthenticated user gets the same flag variant — not a rollout, just a constant.