Tracer bullet: run a Shell Agent Session in a Scratch Workspace inside the API container

[ExtraToast]

Design: #59 (the glossary and ADRs). Use the glossary's terms.

What to build

The thinnest end-to-end path through the new runtime. The user creates a Scratch Workspace in agents-ui, starts a Shell Agent Session, and Attaches to it with a live terminal. The session runs in tmux inside the agents-api container as the agent user. No Pod is created. The gateway's tmux session management and terminal Attach code merges into agents-api, replacing the WebSocket relay to a runner Pod.

Implements ADR 0001: Agent Sessions run inside the API container and ADR 0003: The API and agents run as separate users.

Acceptance criteria

• Creating a Scratch Workspace creates its directory on the workspaces volume, and no Kubernetes API call is made.
• Starting a Shell Agent Session runs a tmux-backed process owned by agent, not api.
• agents-ui Attaches through the existing WebSocket host. Input, output and resize work, and a browser reload reattaches to the same process.
• Stopping the Agent Session ends its process and marks it Stopped.
• An integration test covers create, Attach and Stop against a real tmux.

Blocked by

• Rename Workspace Agent Session to Agent Session and add the Suspended status #60
• Build the agents image with baked tools and separate api and agent users #61