DC-DC Stage 1 (dc-dc-phase0): first real execution + scaffold/tooling fixes ...

First end-to-end run of a DC-DC runbook against real infrastructure (vcloud host,
OpenTofu v1.12.3). Applied 13 libvirt objects (6 DC1 planes, dc1/office1/dc2
pools, office1-local, 3 mesh legs, all MTU 9000); prior VR0 nets torn down (wan
kept as a gap-#17 model); DC2 planes deferred (NetBox supernet), DC2 storage wired.

DOCFIX-178: scripts/prereqs/ idempotent workstation prereq installers + a
  Prerequisites section in the runbook + tests/prereqs/ (25/25). Runbooks must
  not assume prereq runtimes.
DOCFIX-179: OpenTofu scaffold first validation + apply. Per-module
  required_providers (child modules do NOT inherit provider SOURCE -> OpenTofu
  inferred nonexistent hashicorp/libvirt); tofu fmt -recursive; provider "maas"
  deferred to Stage 3 (kept a sensitive key out of Stage-1 plan/state);
  repo-lint now skips the .terraform/ provider cache; .terraform.lock.hcl added.
DOCFIX-180: dc-dc-phase0 runbook as-executed corrections folded in at phase
  close (two MTU domains, non-pristine-host handling, Step-9 13-resource plan,
  -input=false, Known-Gap resolved).

repo-lint 0 fail; prereqs harness 25/25; repo-lint harness 34/34.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>