docs(prd): renumber 0083 -> 0081, and point at the umbrella issue

cf6bbc43 renumbered this PRD 0081 -> 0083 as "0081 already taken", but 0081 is
free: main goes 0080, 0082, and no other branch claims it. 0083 is the number
that is actually taken — feat/pinned-infra-artifacts holds
docs/prds/0083-packaged-infra-artifacts.md, renamed there from its prd-new-
form, so both PRDs were sitting on 0083 and whichever merged second would have
collided. Move back to 0081 and leave 0083 to the infra-artifacts PRD.

The Issue header pointed at #510 (CA) and #512 (git-gate), which have since
been folded into #516 and closed. #516 carries both symptoms plus the shared
root cause, so it is the reference that stays useful.

requirements.gateway.lock also matches "0083" — inside a sha256 hash, left
alone.

docs/prds/0081-reprovision-gateway-state-on-bringup.md

@@ -1,9 +1,9 @@
-# PRD 0083: Reprovision gateway-dependent state on gateway bring-up
+# PRD 0081: Reprovision gateway-dependent state on gateway bring-up
 
 - **Status:** Active
 - **Author:** claude
 - **Created:** 2026-07-26
-- **Issue:** #510, #512
+- **Issue:** #516
 
 ## Summary