feat(plugins): add default start commands for better discoverability

[abueide]

Summary

Adds convenience devbox run commands to all plugins for better out-of-the-box developer experience:

Android Plugin:

• devbox run start:android - Full workflow (start emulator, build, deploy)

iOS Plugin:

• devbox run start:ios - Full workflow (start simulator, build, deploy)

React Native Plugin:

• devbox run start:metro - Start Metro bundler
• devbox run stop:metro - Stop Metro bundler
• devbox run start:android - Full Android workflow
• devbox run start:ios - Full iOS workflow

Problem

Previously, users had to:

1. Manually define these commands in every project's devbox.json
2. Remember the CLI script names (android.sh run, ios.sh run, metro.sh start)
3. Commands weren't discoverable via devbox run --list

Solution

Plugin-Provided Defaults

Commands are now built into the plugins and work immediately for simple projects:

# Works out-of-the-box (just needs build:android or build script defined)
devbox run start:android

# Also works
devbox run start:ios
devbox run start:metro

Still Overridable

Projects can override for custom workflows:

{
  "shell": {
    "scripts": {
      "start:android": [
        "process-compose -f dev-android.yaml --tui=${DEVBOX_TUI:-false}"
      ]
    }
  }
}

Clear Error Messages

Already implemented in the CLI scripts:

When no build script exists:

ERROR: No build:android or build script found in devbox.json.
Define a build script using native tools (e.g., gradle assembleDebug).

When no APK/.app artifact found:

ERROR: No APK found. Searched: ANDROID_APP_APK env var, project root, current directory.
Set ANDROID_APP_APK in devbox.json env, or pass a path: android.sh run /path/to/app.apk
See: plugins/android/REFERENCE.md for APK resolution details.

Build Detection

The commands automatically detect and use:

1. Platform-specific scripts first: build:android, build:ios
2. Generic fallback: build
3. Clear error if neither exists

Benefits

1. Better discoverability - Commands appear in devbox run --list
2. Consistent UX - Same command names across all projects using the plugins
3. Works immediately - No config needed for simple projects with build scripts
4. Still flexible - Full override capability for complex workflows
5. Clear errors - Helpful messages guide users when build scripts or artifacts are missing

Testing

• Error messages verified in android.sh run and ios.sh run
• Build detection logic already implemented (tries platform-specific, then generic)
• README files updated with new command examples
• Will test in analytics-react-native after merge

Documentation Updates

• Updated plugins/android/README.md quickstart
• Updated plugins/ios/README.md quickstart
• Updated plugins/react-native/README.md with full command reference

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