GH-2356: Add the AI issue labeler thin caller

[magicsunday]

Summary

Adds .github/workflows/issue-labels.yml, a thin caller for the reusable magicsunday/.github ai-issue-labeler.yml workflow. This is part of the rollout described in that workflow's own pilot (magicsunday/.github#60), which was verified end-to-end there before this rollout started, and the fully-qualified cross-repo uses: form has already been verified live in several other rollout repos.

Changes

• New file, 31 lines, mirrors the piloted caller's shape exactly except for the uses: reference, which is the fully-qualified cross-repo form (magicsunday/.github/.github/workflows/ai-issue-labeler.yml@main) documented in that workflow's own "CALLER REQUIREMENTS" header block for a caller outside magicsunday/.github itself.
• needs-triage label already present on this repo.
• ANTHROPIC_API_KEY repository secret already provisioned.

Test plan

• YAML valid
• Reviewed against the reusable workflow's own documented caller contract (permissions, secret name, uses: form)
• Live end-to-end test (real issue, confirm a label gets applied) - planned right after merge

[coderabbitai]

Important

• 🔍 Trigger review

This repository does not receive automatic reviews because it has fewer than 10 stars.

⚙️ Run configuration

Configuration used: Repository: magicsunday/coderabbit/.coderabbit.yaml

Review profile: ASSERTIVE

Plan: Team

Run ID: faffbd4d-0f9f-4342-a40b-bd06085897ff

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[magicsunday]

Blocked by a pre-existing, unrelated CI break: #2358 (Rector step fails on build (8.4)). Not caused by this one-file addition; leaving this PR open until #2358 is resolved.