Add compatible structured output to GitHub PR review

## Problem
`github-pr-review` has no structured output, so CI and agents cannot consume PR review results directly.

## Goal
`github-pr-review` emits the same schema as #69, with PR-appropriate fields populated.

## Scope
- Produce the structured document for a PR review.
- Populate PR head SHA and decision consistently with the human output.
- Keep field meaning identical to the local output.

## Non-goals
- Publishing a GitHub status/check. That is a different mechanism — a pass/fail commit status, tracked by #49 (`review-status-enforcement.md`) — not a findings document. Don't conflate the two.

## Acceptance criteria
- PR review can emit a schema-valid document.
- Shared fields match local-review semantics.
- PR head SHA and decision are populated.

## Dependencies
Depends on #67, #68 (both open). Aligns with #69. Blocks #71. Parent: #44.

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

Start by reading dependencies #67 and #68, then compare the local schema and semantics described in #69. Use the GitHub PR Skill validation suite and contract tests from #71 to verify a schema-valid PR review document, including the PR head SHA and decision, with shared fields matching local-review behavior.

Written by the indexing model from the issue text.