`runner install` skips release bundle download when CLI-only binary exists

runner install checks if /var/lib/miren/release/miren exists (line 73) and skips the release download if it does. But the bash installer (api.miren.cloud/install) only installs the CLI binary, not the full base release (containerd, runc, shims). So when the startup script runs the bash installer first (step 50) and then runner install, the install sees the binary and skips the download. The runner then crash-loops with "containerd binary not found."

Found on miren-toys-runner-2 during MIR-965 validation. The check should verify the full release is present (e.g. check for containerd), not just the miren binary.