P2: Remove Vite plugin from `@openelement/content` root export

[SisyphusZheng]

What to build

packages/content/src/index.ts:4 re-exports openContent from vite.ts. Keep the root export runtime-free and move the Vite plugin to @openelement/content/vite.

Acceptance criteria

• Root export no longer pulls in Vite types.
• content/vite subpath exists and is documented.
• Imports updated in adapter-vite and www.

Blocked by

None