Document one end-to-end managed-agent migration between providers

Goal

Add the first reproducible portability walkthrough using a real source and target provider.

The walkthrough should show

• exporting or recreating the source agent as agents.yaml;
• reviewing the declaration in Git;
• changing the target provider and model mapping;
• the exact validate and plan output;
• native, emulated, and unsupported differences;
• applying and running one equivalent session on the target;
• cleanup steps and cost/security caveats.

Start by proposing the provider pair in Discussion #8. Do not commit credentials, state files, workspace identifiers, or unredacted provider payloads.

从 Discussion #8 开始，提出并选择 provider 对。然后使用 agents.yaml 记录迁移过程，包括 validate 和 plan 的确切输出、provider 与 model 的映射、可移植性差异、apply 和 session 步骤、清理工作，以及成本和安全注意事项。只有在无需凭据、状态文件、工作区标识符或未脱敏的 provider payload 的情况下即可复现该演练时，才算完成。