Update Buildkite pipeline for the new JuliaGPU cluster

[maleadt]

The JuliaGPU Buildkite agents have moved to a dedicated cluster, where the single juliagpu queue has been split into per-backend queues. Steps now select agents using queue: "cuda", queue: "rocm" or queue: "oneapi" instead of queue: "juliagpu" combined with a cuda/rocm/intel tag.

This change only affects agent selection; the steps themselves are unchanged.

Since queues are now scoped to a cluster, a single pipeline can no longer mix JuliaGPU and juliaecosystem jobs: pipeline.yml now contains only the GPU jobs, while the juliaecosystem steps have moved unmodified to pipeline-julia.yml, to be uploaded by a separate pipeline in the ecosystem cluster once that is back online.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.31%. Comparing base (b179e7d) to head (5bca51d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #710   +/-   ##
=======================================
  Coverage   52.31%   52.31%           
=======================================
  Files          22       22           
  Lines        1688     1688           
=======================================
  Hits          883      883           
  Misses        805      805           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.