compute-driver: treat openshell-supervisor's command line as a versioned driver interface #4642

Problem

External compute drivers launch openshell-supervisor themselves, so its flags and environment are part of the driver contract. v0.1.4 replaced --parent-liveness-fd <fd> with --parent-liveness-stdin. The release notes don't mention it, and a driver built for v0.1.3 fails against a v0.1.4 supervisor with only a usage error.

Today

External drivers pin one OpenShell release and check openshell-supervisor --version at startup.

Proposal

Either of:

• list driver-facing supervisor flag and environment changes in the release notes
• give the supervisor a machine-readable interface version (for example in --version output or a capabilities subcommand) that drivers can check