Update the Spider worker base image when Spider has a stable release

[coderabbitai]

Summary

Once Spider provides a stable worker-image release, replace the mutable default main tag used by the CLP Spider worker image with that stable, reviewed reference.

Rationale

ghcr.io/y-scope/spider/worker:main can change independently of CLP. Moving to a stable Spider release will make the worker-image dependency deliberate and reproducible while retaining the existing build-argument override.

Affected area

• tools/docker-images/clp-spider-worker/Dockerfile
  • Default SPIDER_WORKER_IMAGE build argument

Acceptance criteria

• Spider has published a suitable stable worker-image release.
• The Dockerfile default no longer references the mutable main tag and instead uses the selected stable, reviewed reference.
• The SPIDER_WORKER_IMAGE build-argument override remains supported.
• The worker image build is validated against the selected Spider release.

Requested by: @20001020ycx

Related pull request: #2408
Related review comment: #2408 (comment)