feat(agent): PR scope creep check in pr_review workflow

**Context:** ROADMAP.md → Agent quality → PR scope creep check (`pr_review`)
**Related:** pr-risk-classifier draft, #425

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## Component

Agent (Python runtime)

## Describe the feature

Advisory-first scope analysis in `coding/pr-review-v1` comparing **declared intent** (task description / issue / PR narrative) to the **actual diff** and touched areas.

Structured output:

| Field | Values / content |
|-------|------------------|
| `scope_rating` | `within_scope`, `mild_expansion`, `significant_expansion`, `likely_scope_creep` |
| `confidence` | numeric or enum |
| `rationale` | files touched, API/schema/config changes, unrelated dependency churn |

**Rollout:** non-blocking reviewer guidance first; optional policy gates for high-risk repos later.

## Use case

Agents often expand beyond the stated task. Reviewers need early signal before deep review. Teams want consistent scope discipline without blocking low-risk expansions by default.

## Proposed solution

1. Extend `pr_review` workflow step to invoke scope analysis after diff is available.
2. Post structured comment via GitHub Reviews API (advisory section).
3. Emit `scope_analysis` event to `TaskEventsTable`.
4. Unit tests with golden fixtures for rating boundaries.
5. Document prompt and output schema in `docs/design/WORKFLOWS.md`.

## Other information

- Design context: `docs/design/EVALUATION.md`, `agent/workflows/`.

- [ ] This might be a breaking change

Beginne mit dem `pr_review`-Workflow in `agent/workflows/` und dem Designkontext in `docs/design/EVALUATION.md` und `docs/design/WORKFLOWS.md`; prüfe, wie der Diff verfügbar wird und wie `TaskEventsTable`-Ereignisse ausgegeben werden. Die Arbeit ist abgeschlossen, wenn Scope-Bewertungen, eine beratende Review-Ausgabe, ein `scope_analysis`-Ereignis, Golden-Boundary-Fixtures und eine dokumentierte Prompt/Schema-Unterstützung den vorgeschlagenen Ablauf unterstützen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.