test(maestro): update tests for async structured spawn

Tests now assert `Starting` after create and use `wait_for_ready`
before checking post-handshake state. Unsupported command test
expects `BadRequest` instead of a persisted Dead session.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/SIDEBAR.md

@@ -160,6 +160,7 @@ The state word is never spelled out — the colour carries it.
 | ---------------- | --------- | ---------------- |
 | `Active`         | `--live`  | `active`         |
 | `Idle`           | `--muted` | `idle`           |
+| `Starting`       | `--muted` | `idle`           |
 | `AwaitingInput`  | `--warn`  | `awaiting`       |
 | `Error`          | `--danger`| `error`          |