chore(railway): add sandbox-agent preview deployment

[mmabrouk]

Context

Railway previews need the sandbox-agent service to run beside API/services and receive the same Daytona credentials used by the agent runtime. Without a Railway slice, the POC can be tested locally but not in preview environments.

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

Changes

Adds the Railway deploy workflow wiring for the sandbox-agent Daytona key, adds a Railway service Dockerfile wrapper for agenta-sandbox-agent, and updates the Railway scripts so configure, deploy, bootstrap, and preview-env resolution include the runner service. The Railway self-host guide now documents the sandbox-agent service in the Railway deployment path.

Tests / notes

• Split from the previously validated working tree. I did not rerun a Railway preview deploy after the branch split.
• The shared GHCR image build workflow is intentionally in the image/CI PR, not this Railway PR.

What to QA

• Run a Railway preview deploy for this stack. The sandbox-agent service should be created and receive SANDBOX_AGENT_DAYTONA_API_KEY.
• Open the preview app and run an agent workflow that uses Daytona-backed code execution. The request should route from services to the sandbox-agent service, not to an in-process fallback.

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

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