refactor(vm): remove legacy openshell-vm crate

[drew]

Summary

Remove the excluded legacy openshell-vm crate now that openshell-driver-vm is the supported VM path.

Related Issue

N/A

Changes

• Removed crates/openshell-vm and its standalone runtime scripts/tests.
• Removed the workspace exclude entry and the unused openshell_vm_base_dir helper.
• Updated architecture, RFC, AGENTS, and VM driver docs to point only at openshell-driver-vm.
• Removed stale k3s setup pins/comments from VM runtime infrastructure.
• Removed explicit VM rootfs cleanup/tests for legacy k3s, kubectl, Rancher, chart, and manifest artifacts.

Testing

• cargo metadata --no-deps --format-version 1
• cargo check --workspace
• cargo test -p openshell-driver-vm
• mise run pre-commit passes
• mise run test
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)