feat: cache local Expo builds on disk

[MounirDhahri]

This PR resolves [making local native builds fast by caching them]

Description

Adds an Expo build cache provider so yarn ios / yarn android reuse a previously built native binary instead of recompiling from scratch when nothing native changed — mirroring the approach in Energy.

• app.json — adds expo.buildCacheProvider using expo-build-disk-cache, caching to node_modules/.expo-build-disk-cache.
• package.json — adds expo-build-disk-cache (devDependency).
• Docs — AGENTS.md (Expo Prebuild) and docs/getting-started.md document the disk cache.

How it works: expo run:* computes a fingerprint of the native project; if a cached build with the same fingerprint exists on disk, it's installed directly, skipping the Gradle/Xcode build. Change a native dependency or app.json and the fingerprint changes, triggering a fresh build.

Disk-only for now, per the request — the cache lives under node_modules/ (already gitignored). expo-build-disk-cache also supports a remotePlugin (e.g. an S3 provider, as Energy uses) which can be layered on later without changing call sites.

CI impact: none — CI builds via expo prebuild + Gradle directly (not expo run), so the provider isn't exercised there; type-check/test/lint are unaffected.

PR Checklist

• I have tested my changes on iOS and Android.
• I have included screenshots or videos, or I have not changed the UI. (build tooling only)
• I have added tests, or my changes don't require any.
• I added an [app state migration], or my changes do not require one.
• I have documented any follow-up work that this PR will require, or it does not require any. (optional remote cache provider later)

To the reviewers 👀

• I would like at least one of the reviewers to run this PR on the simulator or device. (worth a local yarn ios/yarn android twice to see the cache hit)

🤖 Generated with Claude Code

https://claude.ai/code/session_01KPWjhmaeq8DnsiiFD9Eg7A

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