test(e2e): run gpu workloads from manifest

[elezar]

Summary

This PR adds manifest-driven GPU workload execution tests on top of the workload image artifacts from #1484. It keeps the existing GPU device-selection coverage, adds workload execution coverage under the umbrella gpu target, and documents how to build workload images locally before running the GPU e2e suite.

Related Issue

Closes #1472

Changes

• Switch GPU workload execution tests from a single image env var to a YAML workload manifest consumed by the Rust e2e harness.
• Run the manifest-defined workloads through openshell sandbox create --gpu --from <image> -- <command> and enforce declared pass or fail expectations.
• Load the local manifest from e2e/gpu/images/.build/workloads.yaml by default, with OPENSHELL_E2E_WORKLOAD_MANIFEST available for external manifests.
• Update the Docker GPU e2e wrapper to point users at the workload manifest flow when no local manifest exists.
• Add serde_yaml to the e2e crate for manifest parsing.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Validation run:

• mise run e2e:workloads:build
• mise run e2e:docker:gpu

Notes:

• mise run pre-commit currently fails in python:proto with ModuleNotFoundError: No module named 'grpc_tools'; this failure is not caused by the workload-manifest changes.
• Build workload images and generate the local manifest with mise run e2e:workloads:build before running mise run e2e:docker:gpu locally.
• External catalogs can be exercised by setting OPENSHELL_E2E_WORKLOAD_MANIFEST=/abs/path/to/workloads.yaml.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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