feat(remote): warn when the control plane version differs

[outofcoffee]

Every spinloop remote command now warns on stderr when the control plane was deployed by a different spinloop version, without an extra API call. Closes #213.

Summary

• Every control plane Lambda response, errors included, carries an x-spinloop-control-plane-version header.
• spinloop remote bootstrap stamps the control plane with its own version, through SPINLOOP_CONTROL_PLANE_VERSION and a new controlPlaneVersion CDK setting.
• The CLI reads the header on the response to each call and prints one warning per process naming both versions and suggesting spinloop remote bootstrap.
• No warning when the header is absent (older control plane) or when either side is dev. Stdout and exit status are unchanged.

Implementation details

The version is a header rather than a body field because every Lambda builds responses through one helper, and bodies differ in shape between Lambdas. A control plane deployed by hand with pnpm run deploy reports dev and never triggers the warning.

Includes the OpenSpec change warn-on-control-plane-version-mismatch, which still needs archiving before merge.