Add tests, docs, and examples for @varlock/expo-integration

[Copilot]

The Expo/React Native Babel plugin lacked tests, a docs page, and the surrounding package artifacts that every other integration has.

Tests (packages/integrations/expo/test/babel-plugin.test.ts)

14 Vitest unit tests driving the plugin's visitor directly via lightweight mock objects (no @babel/core runtime needed). Covers all decision branches:

• Module init side-effects (execSyncVarlock, initVarlockEnv, patchGlobalConsole called at startup)
• Static replacement for every value type: string → StringLiteral, number → NumericLiteral, boolean → BooleanLiteral, null → NullLiteral, undefined → Identifier('undefined'), object → JSON.parse(...) CallExpression
• Sensitive keys (isSensitive: true) are never passed to replaceWith
• Computed access (ENV["KEY"]), unknown keys, and non-ENV objects are all skipped
• Plugin metadata: correct name, api.cache(false) called

Added test / test:ci scripts to package.json consistent with other packages.

Docs (packages/varlock-website/src/content/docs/integrations/expo.mdx)

New docs page following the astro.mdx / vite.mdx pattern:

• Setup steps (install, varlock init, babel.config.js diff)
• Compile-time replacement explained with before/after bundle example
• Type-safety via @generateTypes
• Multi-environment management with EAS build script examples
• Sensitive value handling with bundle security callout

Also:

• Added Expo / React Native entry (with new badge) to the sidebar in astro.config.ts
• Added Expo to integrations/overview.mdx

Package housekeeping

• Added CHANGELOG.md (present in every other integration package)
• Fixed incorrect import in README.md: import ENV from '@varlock/env' → import { ENV } from 'varlock/env'

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