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.

先從 agents validate CLI 流程及其契約測試開始，接著比較 plan、apply、工作階段和部署在存取網路前如何解析憑證。完成的標準是：新產生的專案支援文件中說明的離線驗證，能區分缺少的變數與 schema 或參照錯誤，為其他命令保留憑證檢查，並更新英文和中文入門文件。