fix(cli): force exit after render to prevent process hang (#104)

## Summary

- Adds `process.exit(0)` after render completes in the CLI `render` command
- Fixes the process hanging indefinitely after `npx hyperframes render --output video.mp4`
- Root cause: Node.js `fetch()` keep-alive pool (from update checks and telemetry) keeps TCP connections open, preventing the event loop from draining
- Telemetry is preserved — the `exit` handler in `cli.ts` calls `flushSync()` which spawns a detached child process

## Test plan

- [x] Run `npx hyperframes render --output test.mp4` and verify the process exits after completion
- [x] Verify telemetry events are still sent (check PostHog)

packages/cli/src/telemetry/client.ts

@@ -105,7 +105,7 @@ export async function flush(): Promise<void> {
   try {
     await fetch(`${POSTHOG_HOST}/batch/`, {
       method: "POST",
-      headers: { "Content-Type": "application/json" },
+      headers: { "Content-Type": "application/json", Connection: "close" },
       body: JSON.stringify({ api_key: POSTHOG_API_KEY, batch }),
       signal: controller.signal,
     });