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

Read ROADMAP.md and docs/design/INTERACTIVE_AGENTS.md, especially the deferred Phase 3 items and the smart progress work. First evaluate the need during dogfood and compare it with agent-authored progress_report events; done is a decision to pursue or defer the optional status-summary design before implementation.

Written by the indexing model from the issue text.