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.

Start with the agents validate CLI flow and its contract tests, then compare how plan, apply, sessions, and deployments resolve credentials before network access. Done means a freshly generated project supports documented offline validation, distinguishes missing variables from schema or reference errors, preserves credential checks for other commands, and updates the English and Chinese getting-started docs.