Define Review Context schema

## Problem
`local-code-review` has a structured, tested `ReviewContext`-shaped reference module (`tests/reference/review/review_context.py`, exercised by `tests/unit/review/test_review_context.py`) matching the illustrative shape in `shared/policies/review-context.md`'s "Recommended internal normalization" section. `github-pr-review`'s own `skills/github-pr-review/policies/review-context.md` describes the same shape only in prose — there is no equivalent structured reference module for it (re-verified: no `dataclass`/`schema` in that policy file, and no github-context reference module under `tests/reference/review/`).

## Goal
An equivalent structured, tested Review Context representation for `github-pr-review`, or an explicit demonstration that the existing shared dataclass/shape validates directly for GitHub-sourced context (e.g. PR description used as context).

## Scope
- Structured representation of GitHub-side review context, matching the shape already defined in `shared/policies/review-context.md`.
- Example GitHub-sourced context documents (e.g. PR description, linked issue) that validate against it.
- Reuse the existing shared shape; do not invent a divergent one.

## Non-goals
- Fetching or resolving external references.
- Any change to the local-side `review_context.py` module or its policy.

## Acceptance criteria
- [ ] A structured Review Context representation exists for `github-pr-review` (new module, or proven reuse of the local one).
- [ ] Example GitHub-sourced context documents validate.
- [ ] Shape stays identical to the shared schema — no divergence between Skills.

## Validation
- Schema lint / example validation, mirroring `test_review_context.py`'s approach.

Start by reading tests/reference/review/review_context.py and tests/unit/review/test_review_context.py, then compare their shape with shared/policies/review-context.md and skills/github-pr-review/policies/review-context.md. Check how GitHub-sourced examples such as a PR description or linked issue could be validated. Done means github-pr-review has an equivalent structured representation or documented direct reuse, with validating examples and no schema divergence.

Written by the indexing model from the issue text.