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

[jeremyeder]

Summary

• Replace all 5 actions/github-script@v8 blocks with gh CLI equivalents, eliminating the JavaScript dependency entirely
• Remove ~40 lines of retry/backoff infrastructure (retryWithBackoff, safeComment) — if gh fails, the step fails, and the existing "Report failure" step handles notification
• Simplify "Check for existing PR" to use gh pr list --search instead of complex jq regex matching

Net reduction: 296 deletions, 137 insertions (-159 lines, from 656 → 496)

What changed

Step	Before	After
Extract issue details	actions/github-script@v8 with Octokit + regex parsing	gh issue view + jq
Check for existing PR	Complex jq regex test("Closes #N($|[^0-9])")	gh pr list --search "Closes #N"
Report no changes	35-line JS block	gh issue comment
Validate changes	JS execFile('git', ['diff']) + Octokit comment	Shell git diff --stat + gh issue comment
Create/update PR	170-line JS with retry/backoff	gh pr create / gh pr comment (~60 lines)
Report failure	45-line JS block	gh issue comment (~15 lines)

What did NOT change

• Trigger configuration (on: issues/issue_comment)
• Permissions block (including id-token: write)
• Determine Amber action type step
• Create Amber agent prompt step
• Create or checkout feature branch step (with all validation)
• Claude Code execution step
• Check if changes were made step

Test plan

• Verify YAML is valid (python3 -c "import yaml; yaml.safe_load(open(...))" — passed)
• Verify actionlint passes (all findings are pre-existing shellcheck info — passed)
• Verify no actions/github-script references remain (grep — passed)
• Label a test issue with amber:auto-fix and verify full flow
• Trigger on an issue that produces no changes → verify "no changes" comment
• Re-label same issue → verify existing PR detection and update comment

Closes #600

🤖 Generated with Claude Code

[jeremyeder]

refactor. features rename the same.