fix(ci): repair issue claim concurrency

[openai0229]

Related issue

Closes #1901

Summary

Removes the unsupported job-level concurrency queue key that caused issue cleanup runs to fail before runner startup. Cleanup runs are isolated per issue while command processing and scheduled sweeps remain globally serialized. Adds a regression test for the supported workflow shape.

Affected surfaces

• Frontend / Web
• Backend / API / Storage
• Database plugin / Driver
• JCEF / Desktop packaging
• CI / Build / Release
• Documentation only

Verification

• Commands and results: rtk node --test script/github/issue-claim.test.js - 27 passed, 0 failed; rtk ruby -e YAML.safe_load(...) - passed; rtk git diff --check - passed.
• Manual verification: Confirmed the workflow contains no queue key and cleanup concurrency includes the issue number.
• UI evidence: N/A

Risk and compatibility

• Public API or stored data: N/A; workflow-only change.
• Database or driver compatibility: N/A; no runtime product code changed.
• Network, privacy, or security: Existing GITHUB_TOKEN permissions are unchanged.
• Community / Local / Pro boundary: Community repository automation only.
• Backward compatibility: Supported GitHub Actions concurrency behavior is restored; command and sweep serialization remain unchanged.

Reviewer map

• Start here: .github/workflows/issue-claim.yml concurrency blocks.
• Failure condition: Any issue event run fails before runner startup, or concurrent cleanup for separate issue numbers blocks unexpectedly.
• Rollback or disable path: Revert this PR; the issue-claim workflow can also be disabled from GitHub Actions.

Contributor declaration

• I linked the Issue that defines this change.
• I tested the affected behavior and reported the actual results above.
• I did not include credentials, private data, or generated build output.
• I disclosed substantial AI assistance below, or this PR contains no substantial AI-generated code.

AI assistance: Codex assisted with diagnosis, implementation, regression coverage, and verification.