refactor: replace actions/github-script with gh CLI in amber-issue-handler

[jeremyeder]

Why

The amber-issue-handler workflow is ~656 lines. Five actions/github-script@v8 blocks contain inline JavaScript (including ~170 lines of retry/backoff infrastructure) that can be replaced with gh CLI commands — cutting the file roughly in half, making it easier to maintain, and eliminating the Octokit JS dependency.

gh CLI is natively available in GitHub Actions runners and well-supported by Claude Code for future modifications.

Changes (in order)

1. Extract issue details — Replace actions/github-script with gh issue view + jq (~30 lines saved)
2. Check for existing PR — Simplify complex jq regex to gh pr list --search (~10 lines saved)
3. Report no changes — Replace actions/github-script with gh issue comment (~20 lines saved)
4. Validate changes — Replace actions/github-script + execFile with git diff --stat + gh issue comment (~25 lines saved)
5. Create or Update PR — Replace actions/github-script (including retryWithBackoff/safeComment infrastructure) with gh pr create + gh pr edit --add-label + gh issue comment (~110 lines saved)
6. Report failure — Replace actions/github-script with gh issue comment (~25 lines saved)

Estimated total: ~220 lines saved, 0 actions/github-script blocks remaining.