minyang2020/@minyang2026/dsh-migrate-on-429

DeepSeek Harness (dsh) plugin: when a session keeps hitting 429 TPM rate limits (typically because the context has grown too long), automatically summarize the current session, hand the task over to a fresh session, and continue there — a true handoff (old session cancelled first), never parallel work. Superset of the old auto-continue behavior (retries a couple of times before migrating).