Allow offline validation before provider credentials are configured

Problem

agents validate performs no provider API calls, but the generated agents.yaml contains ${VAR_NAME} references that currently must resolve before validation completes. A new user therefore cannot validate the generated project until real provider credentials exist.

Desired outcome

Provide a safe validation path that checks YAML structure, references, and provider capability rules without requiring secret values or contacting a provider. Planning and applying must continue to require resolved credentials.

Acceptance criteria

• A freshly generated project can run the documented offline validation flow without real credentials.
• Missing variables are still reported clearly, but are classified separately from schema or reference errors.
• plan, apply, sessions, and deployments continue to fail before network access when required credentials are unresolved.
• CLI contract tests cover both offline validation and credential-required commands.
• English and Chinese getting-started docs describe the final behavior.

Comienza con el flujo de CLI de agents validate y sus pruebas de contrato; después compara cómo plan, apply, las sesiones y los despliegues resuelven las credenciales antes del acceso a la red. Se considera terminado cuando un proyecto recién generado admite la validación offline documentada, distingue las variables faltantes de los errores de esquema o de referencia, conserva las comprobaciones de credenciales para los demás comandos y actualiza la documentación de introducción en inglés y chino.