Add `pnpm native ensure <ios|android>`, which installs a cached dev-client
build keyed by the Expo fingerprint and only runs `expo run` on a miss.
The cache lives in the user cache dir so every worktree shares it, and
app.config.js registers it as Expo's buildCacheProvider so plain
`expo run` reuses hits too. A report-only PR job flags fingerprint
changes against the base in the step summary.
