feat(gateway): support selective compute driver builds

[drew]

Summary

Allow gateway builders to compile any subset of the Kubernetes, Docker, Podman, and VM compute drivers while preserving the existing all-driver default.

The implementation keeps two gateway composition behaviors that were added after the original issue was written: zero-driver protocol-only builds remain valid, and a built-in driver name that is not compiled in can still resolve to an operator-supplied external driver endpoint.

Related Issue

Closes #1943

Direct maintainer authorization was provided despite the issue's stale lifecycle label.

Changes

• Add compute-driver-kubernetes, compute-driver-docker, compute-driver-podman, and compute-driver-vm Cargo features.
• Preserve in-tree-compute-drivers as the default compatibility alias for all four drivers.
• Gate driver registration, configuration, runtime construction, VM process support, and per-driver tracing independently.
• Add feature-aware registry tests and CI coverage for protocol-only, single-driver, and mixed-driver builds.
• Document selective gateway builds and update gateway debugging guidance.

Testing

• mise run pre-commit passes.
• cargo clippy -p openshell-server --all-targets --no-default-features --features <driver> -- -D warnings passes for every individual driver and the protocol-only build.
• Registry tests pass for every individual driver and the Docker+VM mixed build.
• cargo test -p openshell-server --lib passes: 1,463 passed, 8 ignored.
• tasks/scripts/verify-defaults-without-telemetry.sh passes.
• E2E tests added/updated (not applicable; runtime driver behavior is unchanged).

mise run ci was also attempted. Its relevant checks passed, but the local machine's system gateway named default leaks into unrelated gateway-list tests: openshell-cli::completers::gateway_completer_returns_empty_when_no_config and three Go SDK gateway listing tests. These failures reproduce outside this branch's changed packages.

Checklist

• Follows Conventional Commits.
• Commits are signed off (DCO).