ci: build through job-script recipes; hpc lane is hpc-atos

[mcocdawc]

Description

Draft until ecmwf/ci-infrastructure#97 is on main; this uses its run-job-script action.

• The runner build is .ci/build.sh.j2, rendered and run by run-job-script; .github/actions/build-ecbuild goes.
• The recipe extends no shared template: ecbuild compiles nothing. The test kind keeps test-ecbuild.
• execution = "hpc-atos"; the HPC recipe extends cmake-atos.sh.j2 (its new name). Workflows regenerated: -hpc → -hpc-atos.

Reproduce a leg locally: ci-infrastructure-render --list, then ci-infrastructure-render --leg '<job title>' -o ci-job.sh.

🤖 Generated with Claude Code

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• 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.

[mcocdawc]

new CI is green and I merge now.