Author eval suite for agent `azure-requirements-gatherer`

## Agent

`azure-requirements-gatherer` — source: `.github/agents/azure-requirements-gatherer.agent.md`

## Scope

Author the eval suite at `.github/evals/agents/azure-requirements-gatherer/`:

- [ ] `eval.yaml` — suite config (executor, model, graders)
- [ ] At least 2 positive tasks under `tasks/positive-*.yaml`
- [ ] At least 1 negative task under `tasks/negative-*.yaml`
- [ ] Entry added to `.github/evals/manifest.yaml` at `tier: expanded`

## Notes

This agent has an interactive Q&A loop that would deadlock under `copilot-sdk`. The eval MUST exercise the agent's documented headless mode — the task prompt should explicitly set that mode so the agent produces a single non-interactive response.

## Procedure

1. `/agent-bench azure-requirements-gatherer` drafts the suite from the live `.agent.md`.
2. `waza run .github/evals/agents/azure-requirements-gatherer/eval.yaml -v` locally.
3. `/agent-improve azure-requirements-gatherer` to iterate on graders.
4. Open PR.
5. Mock CI runs automatically. A maintainer will dispatch a real-model run before merge.

## Acceptance

- [ ] Suite runs cleanly in `mock` executor.
- [ ] At least one positive task passes in a real-model run.
- [ ] All negative tasks produce a refusal or out-of-scope acknowledgement.
- [ ] `manifest.yaml` entry added; PR description includes the real-model run summary.

## Conventions to follow

- Persona lock: refusal graders should accept the agent's own scope language.
- Prompt graders need `continue_session: true` in their grader config.

## Related

- Umbrella: #93
- Harness: #61