Extend workflow validate with artifact and dependency readiness checks

`comfy workflow validate` now checks an arbitrary workflow against the target catalog, but it does not provide the full readiness intersection available from `templates check`. Extend the arbitrary-workflow path so agents can determine whether the workflow is runnable here without submitting it.

- Check referenced input artifacts and required models/custom nodes.
- Return actionable missing-item metadata, including download guidance when available.
- Keep schema/catalog validation and live-target failure behavior intact.

Paper-code audit evidence

Linear BE-10027 is partially stale against `main@99c5a596`: `comfy workflow validate` exists at `comfy_cli/command/workflow.py:1586-1794`, resolves the live target/catalog, and fails closed when no catalog can be loaded. What remains is parity with the artifact/file/API-dependency intersection that `templates check` provides; the current validator covers classes, input shapes, enums, and wiring.

The canonical validator surface blames to `mattmillerai`. Independently re-verified from scratch for OPP-48 before filing.