[FEAT] add gh_pr_review tool to fetch PR review comments

[four-bytes-robby]

Goal

Add a new tool gh_pr_review that fetches and displays review comments on a GitHub pull request, including approval state and review bodies. Mirrors the existing gh_pr_status / gh_pr_comment patterns.

Background

Currently the supertools plugin can add a PR comment (gh_pr_comment) and check PR status (gh_pr_status), but cannot fetch existing review comments. Code-reviewer subagents and humans need to inspect bot findings (e.g. copilot, codereviewer) without scraping gh pr view via bash — saves ~90% tokens.

Acceptance Criteria

• New file src/tools/gh-pr-review.ts exports ghPrReviewTool
• Tool signature: { pr: number, repo?: string }
• Returns formatted output with: issue-level comments (with author, body, timestamp) + reviews (with author, state, body)
• Filters out empty review bodies (approvals with no comment)
• Truncates long bodies (500 chars for reviews, 300 for comments)
• Handles empty case: "No review comments on PR #N."
• Logs start/success/error via logDebugEvent
• Registered in src/four-opencode-supertools.ts as gh_pr_review
• Build passes: bun run build
• Repo has no other side-effects

Implementation Notes

• Use gh pr view <N> --json comments,reviews — confirmed both fields exist in CLI
• Note: comments field contains issue-level (timeline) comments only; path/line are NOT present (those are inline review comments, separate GraphQL query — out of scope for v1)
• Follow existing pattern from gh-pr-comment.ts: runGh + resolveRepo from ../lib/gh-utils
• Use positional resolveRepo(repo, ctx.directory) — matches the existing call signature in gh-pr-comment.ts

[four-bytes-robby]

Tool exists on main (merged via PR #19, fixed in PR #19)