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

Commencez par le workflow `pr_review` dans `agent/workflows/` et le contexte de conception dans `docs/design/EVALUATION.md` et `docs/design/WORKFLOWS.md` ; examinez comment le diff devient disponible et comment les événements de `TaskEventsTable` sont émis. Le travail est terminé lorsque des évaluations de portée, une sortie de revue consultative, un événement `scope_analysis`, des fixtures de limites dorées et une prise en charge documentée de prompt/schema prennent en charge le flux proposé.

Rédigé par le modèle d'indexation à partir du texte de l'issue.