Before: `exec: "docker": executable file not found in $PATH` — cryptic,
no recovery guidance, workspace row left in broken registered-only state.
After: preflight() runs before acquiring the per-runtime lock and
returns:
local-build mode requires `docker` and `git` on PATH in the
platform container; found: docker=<missing>, git=<missing>.
Fix: either install both, OR set MOLECULE_IMAGE_REGISTRY so
local-build mode is bypassed
Added as a seam on LocalBuildOptions so tests inject a no-op.
Two new tests cover the failure and passthrough paths.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>