Add structured output to local review

## Problem
`local-code-review` emits only the human-readable report.

## Goal
`local-code-review` emits a schema-valid machine-readable result.

## Scope
- Produce the structured document from the same review result.
- Offer it as an opt-in output without replacing the report.
- Include the reviewed SHA and the schema version.

## Non-goals
- GitHub PR wiring (see #70).

## Acceptance criteria
- Local review can emit a schema-valid document.
- Human-readable report unchanged by default.
- Output includes schema version and reviewed SHA.

## Dependencies
Depends on #67, #68 (both open). Blocks #71. Parent: #44.

## Validation
- Contract tests (#71); local Skill validation suite.

Start with the local-code-review entry point and the local Skill validation suite; inspect how the existing review result is produced and how output options are exposed. Done when an opt-in run emits a schema-valid document containing the schema version and reviewed SHA, while the default human-readable report remains unchanged; validate with contract tests (#71).

Written by the indexing model from the issue text.