ci(agent): build and test sandbox-agent images

[mmabrouk]

Context

The POC needs a reviewable slice for image publication and CI, separate from local compose, Railway deploy scripting, and Kubernetes templates. This lets us test the image build path without mixing it into the deployment-specific PRs.

This PR is stacked on #4786 and should be reviewed with the existing agent-workflows POC PRs (#4771, #4772, #4776, #4778, #4779).

Changes

Adds sandbox-agent to the Railway GHCR multi-arch build workflow, wires GH compose examples to consume the published sandbox-agent image, adds the runner service unit-test job, and documents/builds the custom Daytona sandbox image assets. The Railway deploy PR consumes these images but does not own the build workflow.

Tests / notes

• Split from the previously validated working tree. I did not rerun GitHub Actions after creating this PR branch.
• Runner Dockerfile internals that GitButler marks as owned by the runner-engine lane remain in feat(agent): runner engines, HTTP server, tracing, and docker image #4778. This PR owns the image build orchestration and custom sandbox image assets.

What to QA

• Run the image build workflow for a test tag. It should publish agenta-sandbox-agent for amd64 and arm64 and merge a manifest.
• Run the services node unit-test job. It should execute services/agent tests through the new CI path.
• Use the GH compose examples with the published tag. The sandbox-agent container should start from GHCR rather than a local dev build.

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Project	Deployment	Actions	Updated (UTC)
agenta-documentation	Ready	Preview, Comment	Jun 20, 2026 11:40pm

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