Roll out the unrolled agent loop in Desktop with configurable operations

**What problem would this solve?**

The unrolled agent loop is currently selected through the process-wide `GOOSE_STATE_MACHINE` flag. Goose Desktop should be able to adopt it by default without changing the behavior of the CLI or other ACP clients that may use the same backend.

Desktop also needs a way to return to the legacy loop during the rollout. Separately, operators need a way to disable individual operations in the unrolled pipeline without adding Desktop-specific behavior to the backend.

Today, bang-shell input is also a special case that selects the unrolled loop even when the flag is off. That makes the loop choice inconsistent and prevents bang shell from being an unrolled-loop-only feature.

**What would a good outcome look like?**

- Goose Desktop uses the unrolled agent loop by default.
- Desktop has a persisted setting that can switch back to the legacy loop.
- The Desktop choice is sent on each ACP prompt as client-scoped metadata.
- Other ACP clients and the CLI keep their current default behavior.
- Bang shell no longer selects the unrolled loop. It is available only as an operation within that loop.
- Existing state-machine cleanup and lifecycle coverage is preserved before the Desktop rollout is enabled.

**Possible approaches**

Desktop can send an explicit value in ACP prompt metadata:

```json
{
"_meta": {
"goose": {
"unrolled_agent_loop": true
}
}
}
```

Desktop should send `false` when the setting is off rather than omitting the value. Clients that do not know about this setting omit it.

Suggested precedence:

1. An explicit ACP metadata value selects the loop for that prompt.
2. If the metadata is absent, `GOOSE_STATE_MACHINE` remains the existing development/rollout override.
3. Otherwise, the legacy loop remains the default.

The issue involves modifying the Goose Desktop client to send a metadata flag and adjusting the backend's loop selection logic. Start by examining the ACP prompt metadata handling in the Desktop codebase and the agent loop selection in the backend (likely around the GOOSE_STATE_MACHINE flag). Look for existing tests for state-machine cleanup and lifecycle coverage to ensure changes preserve behavior. 'Done' means Desktop can toggle the loop via a setting, metadata is sent correctly, and bang shell no longer forces the unrolled loop.

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