ci: drop the redundant prefix and current-branch

[mcocdawc]

• Once merged (right after Downstream CI by level label; leg-level job-script; hpc-submit runners; no fork checkout ci-infrastructure#92), change the labels (a rename keeps them on open pull requests):

  gh label edit run-downstream-CI --name run-downstream-ci:all -R ecmwf/eckit
  gh label edit downstream-CI-not-needed --name downstream-ci-not-needed -R ecmwf/eckit
  gh label create run-downstream-ci:1 -R ecmwf/eckit
  gh label create run-downstream-ci:2 -R ecmwf/eckit
  gh label create run-downstream-ci:3 -R ecmwf/eckit
  

Description

• Drop prefix, current-branch, [downstream-gate] and the implied needs.
• Level labels from the actions' defaults.
• Regeneration bot (regenerate-workflows.yml).
• job-script/ctest-args in the leg defaults.
• actionlint.yaml: hpc-submit instead of arc-hpc-pet-vsphere-prod.
• ci.yml uses actions/checkout; the label gate runs on pull_request.
• Merge right after Downstream CI by level label; leg-level job-script; hpc-submit runners; no fork checkout ci-infrastructure#92.

🤖 Generated with Claude Code

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌦️ >> Documentation << 🌦️
https://sites.ecmwf.int/docs/dev-section/eckit/pull-requests/PR-356

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.97%. Comparing base (71af966) to head (bbf6386).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #356      +/-   ##
===========================================
+ Coverage    67.95%   67.97%   +0.01%     
===========================================
  Files         1187     1187              
  Lines        62532    62532              
  Branches      4715     4715              
===========================================
+ Hits         42496    42503       +7     
+ Misses       20036    20029       -7     

☔ 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.