Make AI auto-review sticky comments configurable (default on)

Description

The automatic AI code review (ai-automatic-review job in .github/workflows/ai_claude-orchestrator.yml) is hard-pinned to use_sticky_comment: false, so it posts a fresh comment on every push. We want one auto-updating sticky review comment per PR by default, while keeping it toggleable without a code change.

Driven by a repo/org variable AI_REVIEW_STICKY_COMMENTS:

use_sticky_comment: ${{ vars.AI_REVIEW_STICKY_COMMENTS || 'true' }}

• Unset (default) → true → one rolling sticky comment per PR
• AI_REVIEW_STICKY_COMMENTS = false → per-commit fresh comments (old behavior)
• Any other value → truthy → sticky

Small CI/workflow config change; no product code impact.

Acceptance Criteria

• Automatic AI review uses a sticky comment by default (variable unset)
• Setting AI_REVIEW_STICKY_COMMENTS=false restores per-commit fresh comments
• No other review paths (interactive @claude, backend-reviewer) change

Priority

Low

Additional Context

Implemented by PR #36336.

Start in .github/workflows/ai_claude-orchestrator.yml at the ai-automatic-review job and inspect its use_sticky_comment setting. Verify the repo/org variable behavior for unset and false values, while leaving the interactive @claude and backend-reviewer paths unchanged. Done means the automatic review defaults to one sticky comment per PR and can be switched back to fresh comments with AI_REVIEW_STICKY_COMMENTS=false.