Prompt retry after a client timeout has no idempotency key

Deferred from the v0.10.0 release review. Self-disclosed.

A prompt retried after a client-side timeout or connection reset carries no idempotency key (`packages/opencode/src/cli/cmd/run.ts`). If the server accepted the request and only the response was lost, the retry is executed a second time.

For a data tool this is not merely a duplicated read: a `sql_execute` carrying a mutating statement can run twice against a live warehouse. The exposure is largest under automation that layers its own retries on top.

Fixing this needs a server-side idempotency contract, not a client-only change.

Start by reading packages/opencode/src/cli/cmd/run.ts and tracing how a prompt is retried after a client timeout or connection reset. The issue requires an agreed server-side idempotency contract; done means an accepted mutating request cannot execute twice when its response is lost and the client retries.

Written by the indexing model from the issue text.