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.

先从 agents validate CLI 流程及其契约测试入手，然后比较 plan、apply、会话和部署在访问网络前如何解析凭据。完成的标准是：新生成的项目支持文档中说明的离线验证，能够区分缺少的变量与 schema 或引用错误，为其他命令保留凭据检查，并更新英文和中文入门文档。