feat(cli): optional LLM-synthesized status summary

**Context:** ROADMAP.md → LLM-synthesized status summary (optional)

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## Component

Scripts / CLI

## Describe the feature

Optional `bgagent status --summarize` mode where a **Lambda narrates** recent `TaskEvents` **without waking the agent**. Behind a flag; pursue only if agent-authored `progress_report` events are insufficient.

## Use case

Some operators want narrative status before smart progress ships, or for legacy tasks without structured reports. Cost and hallucination risk require opt-in.

## Proposed solution

1. `GET /tasks/{id}/status-summary` Lambda reads events, calls Bedrock, returns cached summary (short TTL).
2. CLI flag; show cost estimate in `--verbose`.
3. Disclaimers in output ("synthesized, not from agent").
4. Rate limits per user/task.
5. Defer until smart progress + ask ship; evaluate need in dogfood.

## Other information

- Does **not** replace smart progress updates.
- Design context: `docs/design/INTERACTIVE_AGENTS.md` Phase 3 deferred items.

- [ ] This might be a breaking change

Leggi ROADMAP.md e docs/design/INTERACTIVE_AGENTS.md, in particolare gli elementi rinviati della Fase 3 e il lavoro sui progressi intelligenti. Valuta innanzitutto la necessità durante il dogfood e confrontala con gli eventi progress_report scritti dagli agenti; il risultato deve essere una decisione se portare avanti o rinviare la progettazione opzionale del riepilogo dello stato prima dell’implementazione.

Scritto dal modello di indicizzazione a partire dal testo della issue.