Constant EPHEMERAL_WORKSPACE_WARNING 

pub const EPHEMERAL_WORKSPACE_WARNING: &str = "\u{26a0}\u{fe0f} EPHEMERAL WORKSPACE: the active runtime uses an ephemeral workspace \
     (tmpfs / no host volume mount). Files written here do NOT persist on the host after this \
     session ends, and reads may return stale or empty data. To make the workspace persistent, \
     set `runtime.docker.mount_workspace = true` in your config and ensure the workspace \
     directory is bind-mounted into the container.";

Loud, actionable banner that filesystem-touching tools surface when the active runtime uses an ephemeral workspace — e.g. a Docker container with no host volume mount, where the workspace is a private tmpfs. In that mode writes succeed inside the container but never reach the host and are discarded when the session ends, and reads may return stale or empty data. Surfacing this prevents the silent data loss reported in issue #4627.

file_write refuses outright (it exists only to persist data). The general-purpose shell, file_read, and file_edit tools stay usable but attach this warning so the agent — and through it the user — knows the workspace is ephemeral and how to fix it.