C4 DONE: document the NetBox sandbox loop; add the upstream-write guard it exposed ...

C4 (Stage 2 close-out) -- the sandbox loop is now documented in two places, no
rationale duplicated: docs/dc-dc-netbox-buildout-scope.md section 8 (design: why a
sandbox, the fidelity check + its 2026-07-14 hardening, the write-guard table, the
WAF trap, C2 preconditions) and runbooks/dc-dc-phase1-office1-standup.md Step 10b
(the dump->seed->apply->verify->gated-write sequence, two-host/two-token separation).
Drafted by a subagent, corrected against this session's own fixes before placement.

The guard C4 exposed: roles-aggregates-import.py and dc-dc-prefixes-import.py had NO
hostname gate -- until today the WAF 403 was the only thing stopping an accidental
--commit to the apex, and the UA fix removed that accidental safety. Both now gate a
non-sandbox --commit behind --yes-write-upstream (SANDBOX_HOSTS allowlist), matching
d115-office-carve.py -- so all three write-capable importers behave identically. This
is defence-in-depth with the whole-plan preflights (which prevent a half-write).

Tests: prefixes-import 86->91 (refused without the flag + writes nothing; allowed
with it; sandbox host needs no flag). roles-aggregates 20->24. GAUNTLET ALL GREEN (58).

Stage 2: C1/C3/C4 DONE. C2 (production write) is the only item left -- gated, with
preconditions now documented (re-verify sandbox under the hardened check, run from
office1-netbox, --yes-write-upstream, approve each mutation).

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