DOCFIX-175/176: OpenTofu state file security + VR1 teardown/rollback runbook ...

Found while reviewing what's missing from the VR1 buildout ahead of
tomorrow's execution.

DOCFIX-175: terraform.tfstate stores sensitive=true variables (like
maas_api_key) in plaintext regardless of the flag, which only redacts
CLI/plan output (verified against HashiCorp's own docs). Added .gitignore
coverage, a "State file handling" section in opentofu/README.md, an inline
caveat on maas_api_key's description, and a "SECURE THE STATE FILE" callout
at Stage 1 Step 10 -- the actual first real tofu apply (an early draft
wrongly assumed Stage 3; corrected after reading the runbooks directly).

DOCFIX-176: new runbooks/dc-dc-teardown-rollback.md for the OpenTofu/
libvirt/MAAS-vm_host layer, which D-061's existing teardown scripts don't
cover (different layer, juju/MAAS-machine). Two paths (scoped -target
teardown or full VR1 destroy), a rollback decision tree favoring
fix-forward over destroy, and explicit mesh-link shared-infrastructure
handling.

Also folded in: a fix to docs/changelog-20260710-ledger-scan-self-inflicted-falsepositive-fix.md
after re-running ledger-scan.sh caught that its own narration (and the
session-ledger mirror) had reintroduced the exact false-positive class
DOCFIX-174 had just fixed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QrcJx8TUar7pYAvpGJw57A

DOCFIX-174: fix self-inflicted false positive in ledger-scan.sh next-free scan ...

DOCFIX-173's own changelog narrated its {3}->{3,} regex fix using a literal
illustrative example ("DOCFIX-1004"), which the widened regex then matched
as a real assignment -- inflating DOCFIX next-free from 174 to 1005.
Reworded the illustrative examples in prose, added a reproduc-line
exclusion to nextfree_mentions() as defense-in-depth, and added a
regression fixture to the test harness (37/37 PASS).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QrcJx8TUar7pYAvpGJw57A