(cli): `bgagent stats` for agent contribution and quality metrics

Component

Scripts / CLI

Describe the feature

Add bgagent stats to report aggregate outcomes over a time window: tasks submitted, PRs opened, merge rate, median revision cycles, CI pass rate, cost totals—filterable by repo and user.

Use case

Operators adopting background agents need outcome metrics, not task counts alone. Roadmap Evaluation pipeline and operator dashboard cover backend aggregation; CLI users and platform teams want a quick, scriptable view for adoption reviews and executive summaries.

Proposed solution

1. GET /stats (or query Tasks table + webhook-derived merge state) returning JSON aggregates.
2. CLI: bgagent stats [--repo] [--since 30d] [--format table|json].
3. Reuse PR outcome signals from roadmap PR outcome tracking when available; v1 can use task terminal states + GitHub API sampling with documented limitations.
4. Document proxy vs. customer-impact metrics (merge rate is proxy; optional manual tags later).

Other information

• Complements roadmap Validation and risk analytics and control panel—not a duplicate if scoped to CLI/API summary.
• Auth: same Cognito constraints as list/status.

Acknowledgements

• I may be able to implement this feature
• This might be a breaking change

Acceptance criteria

• API endpoint documented in API_CONTRACT.md with stable response shape mirrored in cli/src/types.ts.
• CLI prints table and JSON; respects --verbose for query details.
• Tests in cli/test/ with mocked API client.
• User guide explains metric definitions and limitations.

Commencez par examiner les critères d’acceptation dans API_CONTRACT.md et les types de réponse dans cli/src/types.ts, puis inspectez les tests existants du client API de la CLI sous cli/test/. Le travail est terminé lorsqu’un endpoint stats stable et documenté, une sortie sous forme de tableau et de JSON avec les filtres demandés et les détails verbeux, des tests mockés et un guide utilisateur couvrant les définitions et les limites des métriques sont disponibles.