Restores `hooks/misc/useTrackExperimentExposure.ts`, fixing the
typecheck failure on master that broke the latest Studio production
deploy.
The hook was deleted as dead code in #49719 (it was genuinely unused on
master at the time), but #49534 was in flight and reintroduced a usage
in `plan-presentation.ts`. The two PRs merged cleanly with no textual
conflict, so nothing typechecked the combination until the deploy off
master failed with:
```
plan-presentation.ts(5,44): error TS2307: Cannot find module '@/hooks/misc/useTrackExperimentExposure'
```
**Added:**
- `apps/studio/hooks/misc/useTrackExperimentExposure.ts` — restored
verbatim from before #49719; no longer dead code since
`plan-presentation.ts` imports it
## To test
- `pnpm --filter studio typecheck` passes (verified locally)
- `pnpm knip --workspace apps/studio` no longer flags the hook (verified
locally)
- Studio production deploy succeeds once merged
https://claude.ai/code/session_01XZGr2n1dBzJ7m3DYsGu852
Co-authored-by: Alaister Young <10985857+alaister@users.noreply.github.com>