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.

Comece pelo fluxo de CLI de agents validate e seus testes de contrato; depois, compare como plan, apply, as sessões e os deployments resolvem as credenciais antes do acesso à rede. Está concluído quando um projeto recém-gerado oferece suporte à validação offline documentada, distingue variáveis ausentes de erros de esquema ou de referência, preserva as verificações de credenciais para os outros comandos e atualiza a documentação de introdução em inglês e chinês.