[agent-issue]: Allow model-selected reviewers in create-pull-request

### Submission requirements

- [x] I generated this issue with an agent that used `.github/agents/ado-aw.agent.md`.
- [x] I reviewed the generated issue and confirm it is being filed directly in `githubnext/ado-aw`.

### Problem summary

`create-pull-request` supports reviewers only through static workflow frontmatter:

```yaml
safe-outputs:
create-pull-request:
reviewers:
- user@example.com
```

The model-facing `CreatePrParams` accepts title, description, repository, and labels, but not reviewers. This prevents workflows from selecting reviewers dynamically—for example, adding the assignee of the issue that triggered an autofix workflow.

### Reproduction details

1. Create an issue-triggered workflow that opens an Azure DevOps PR.
2. Provide the authoritative issue assignee to the model.
3. Ask the model to add that assignee as a reviewer.
4. The model cannot include reviewers in its `create-pull-request` call.
5. `update-pr` supports dynamic reviewers, but requires a numeric PR ID that does not exist until safe-output execution.

This leaves custom post-processing scripts or hardcoded reviewers as the only options.

Expected: the model can propose reviewers as part of `create-pull-request`, subject to safe-output policy and threat detection.

### Proposed next step

Add an optional `reviewers` field to `CreatePrParams`.

During safe-output execution, merge and deduplicate model-supplied reviewers with statically configured reviewers, then use the existing reviewer identity resolution and PR reviewer APIs.

Consider requiring explicit operator opt-in, such as `allow-agent-reviewers: true` or an `allowed-reviewers` policy. Update the tool schema, documentation, and executor tests accordingly.

Posted by Copilot CLI assistant.