Support explicit local narration in plain watch

## Problem

Plain watch is pipe-oriented output; adding animation or implicit downloads would break its existing consumer contract.

## Proposed change

Provide explicit opt-in narration using the shared session and an already-installed model. Keep default output unchanged; retain line-oriented recommendation association without cursor animation. Adopt the #454 plain-output policy for incremental versus completed admitted text. Model management remains in interactive frontends; plain watch never installs implicitly.

## Acceptance Criteria

- [ ] AC1: A captured default plain-watch replay is unchanged with local AI disabled or unspecified.
- [ ] AC2: An opted-in actual CLI replay emits narration associated with the correct recommendation, without terminal cursor-control animation or blocking normal recommendation lines.
- [ ] AC3: Missing-model, failure, rapid-pick and shutdown CLI checks preserve normal output and do not emit late obsolete narration.
- [ ] AC4: A network-observation check confirms plain-watch opt-in never triggers artifact installation or remote inference.

## Implementation classification

- **Estimated size:** S
- **Orchestration risk:** Normal
- **Reason:** This is a narrow line-oriented consumer of the already-guarded shared narration contract.

## Dependencies

Direct prerequisites: #460. Documentation consumer: Document local narration installation and troubleshooting.