docs: rewrite README with flavors + EAS guide

[pythonsst]

What

Replaces the default React Native boilerplate README with a proper guide so anyone can understand how the build flavors and EAS pipeline are set up.

Covers

• Three environments (flavors) — development / staging / production comparison table (app IDs, API URLs, OTA channels, store tracks, audience)
• How a flavor is defined on each platform — how the same environment lines up across Android productFlavors, iOS schemes, EAS profiles (eas.json), and .env.<environment> files
• Env config — what's in each .env file, with the warning that react-native-config bakes values into the binary (no secrets)
• Running locally — the yarn android:* / yarn ios:* scripts
• EAS build / submit / OTA update — the wrapped yarn eas:* commands
• OTA vs. new build decision table
• CI/CD — the GitHub Actions workflows and EXPO_TOKEN secret
• Release flow + one-time setup, linking to docs/EAS.md

All content is drawn from the actual eas.json, app.config.js, package.json, build.gradle flavors, iOS schemes, and workflow files.

Note

The unrelated project.pbxproj DEVELOPMENT_TEAM change (local Xcode signing identity) is intentionally not included in this PR.

🤖 Generated with Claude Code