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.

從 Discussion #8 開始，提出並選擇 provider 配對。接著使用 agents.yaml 記錄 migration，包括 validate 和 plan 的確切輸出、provider 與 model 的對應關係、可攜性差異、apply 和 session 步驟、清理工作，以及成本與安全注意事項。只有在不需要憑證、狀態檔案、工作區識別碼或未編輯的 provider payload 的情況下即可重現此 walkthrough，才算完成。