Go build fails on Linux: VZ symbols referenced without a darwin build tag

[Snider]

Go Tests + Coverage and golangci-lint have failed on every main run since at least 2026-07-31. Not caused by any open PR.

pkg/agentic/dispatch_vz.go:101:60: undefined: container.NewVZProvider
pkg/agentic/dispatch_vz.go:270:44: undefined: container.ExecResult
pkg/agentic/dispatch_vz.go:288:36: undefined: container.ExecResult
pkg/agentic/runtime_container.go:59:19: undefined: container.IsVZAvailable
FAIL dappco.re/go/agent/cmd/core-agent [build failed]
FAIL dappco.re/go/agent/pkg/agentic  [build failed]
FAIL dappco.re/go/agent/pkg/brain    [build failed]
FAIL dappco.re/go/agent/pkg/monitor  [build failed]
FAIL dappco.re/go/agent/pkg/runner   [build failed]
FAIL dappco.re/go/agent/pkg/setup    [build failed]

VZ is Apple's Virtualization.framework, so container.NewVZProvider, container.ExecResult and container.IsVZAvailable only exist in dappco.re/go/container on darwin. Neither go/pkg/agentic/dispatch_vz.go nor go/pkg/agentic/runtime_container.go carries a //go:build darwin constraint, so Linux CI compiles them and cannot resolve the symbols. It builds on a Mac, which is why this was not caught locally.

Fix is a build-tag split: //go:build darwin on the VZ implementation plus a //go:build !darwin stub providing the same internal surface (containerRuntimeAvailable / runtimeUsesProvider returning false for RuntimeVZ). Worth checking whether container.ExecResult is genuinely darwin-only upstream or should be portable — if the latter, the fix belongs in dappco.re/go/container.

Blocks merge for every PR while main is red.