docs: renumber PRD 0081 -> 0083 across the reconcile chunk

Follows the base PR's renumber (0081 was already taken; main is on 0082). Update
every `PRD 0081` reference in the gateway-attach code, tests, and ADR 0006, plus
ADR 0006's link to the PRD file. No behaviour change.

Refs #516, #519.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/backend/gateway_attach.py

@@ -1,4 +1,4 @@
-"""The shared gateway-attach reconcile flow (PRD 0081).
+"""The shared gateway-attach reconcile flow (PRD 0083).
 
 `BottleBackend.attach_bottled_agents_to_gateway()` delegates here so every
 backend reconciles running bottles against a freshly-booted gateway *the same
@@ -21,7 +21,7 @@ if TYPE_CHECKING:
 @dataclass(frozen=True)
 class GatewayAttachResources:
     """Everything a running bottle needs to (re)attach to the current gateway on
-    a cold boot (PRD 0081). Gathered once per reconcile and handed to every
+    a cold boot (PRD 0083). Gathered once per reconcile and handed to every
     bottle. The CA now; egress secrets and git-gate state join as the reconcile
     grows (#516)."""