feat(core): add Magek.init for programmatic environment setup

[o-mid]

Summary

Magek.config was built when @magek/core loaded, so MAGEK_ENV had to be set before the import. Config is now created the first time it is read. Magek.init(environment) sets the name before that read. If init is not called, MAGEK_ENV is still used.

init has to run before any module that uses Magek decorators or Magek.configure is evaluated. ES import declarations are hoisted, so a call under those imports in the same file is too late.

Calling init again with the same name does nothing. A different name throws, including when config was already built from MAGEK_ENV.

Semver: the version policy's next bump is still patch. This adds Magek.init and keeps the MAGEK_ENV path. It is not a silent upgrade for tests: Magek.config is a getter, and sinon replace(Magek, 'config', config) throws. Use replaceGetter(Magek, 'config', () => config). docs/content/advanced/testing.md shows that.

Fixes #664

Test plan

• cd packages/core && rushx test (337 passing)
• cd packages/core && rushx lint:fix
• cd packages/server && rushx test (39 passing)