feat: retry model capacity failures

[nikita-ashihmin]

Summary

• retry the exact Selected model is at capacity failure with five jittered backoff windows: 1–10s, 1–30s, 30–60s, 60–120s, and 120–300s
• publish each scheduled retry through ACP as a warning and continue in a fresh turn without replaying the original prompt
• preserve cancellation and plan-implementation lifecycle behavior, then return the terminal failure after retries are exhausted

Testing

• npm run typecheck
• npm run build
• npm test -- --run (459 passed, 28 skipped)