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

Follows the base branch back to 0081. 0083 was already claimed by
feat/pinned-infra-artifacts (docs/prds/0083-packaged-infra-artifacts.md), so
the two PRDs would have collided on whichever merged second; 0081 is free.

29 references across 21 files — the "PRD 0083" prose in the backends, the
shared gateway_attach flow, the tests, and the ADR — plus the PRD filename in
the ADR's reference link, which now resolves again.

requirements.gateway.lock matches "0083" inside a sha256 hash and is
deliberately untouched.

Unit suite unchanged: same 13 pre-existing failures as main
(test_cli_start_selector, test_firecracker_cleanup).

bot_bottle/backend/base.py

@@ -528,7 +528,7 @@ class BottleBackend(ABC, Generic[PlanT, CleanupT, AttachTargetT]):
 
     def attach_bottled_agents_to_gateway(self) -> None:
         """Reconcile every running bottle against the freshly-(re)booted gateway
-        on the cold-boot bring-up path (PRD 0083). The shared flow + fail-hard
+        on the cold-boot bring-up path (PRD 0081). The shared flow + fail-hard
         policy live in `gateway_attach`; backends override only the three
         primitives below (ADR 0006)."""
         from .gateway_attach import reconcile_running_bottles