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/docker/backend.py

@@ -54,7 +54,7 @@ class DockerBottleBackend(BottleBackend["DockerBottlePlan", "DockerBottleCleanup
 
     def __init__(self, *, infra: "DockerInfraService | None" = None) -> None:
         # The infra service whose gateway just cold-booted, passed in on the
-        # bring-up reconcile path (PRD 0083) so `_gateway_attach_resources`
+        # bring-up reconcile path (PRD 0081) so `_gateway_attach_resources`
         # reads THAT gateway's CA rather than a fresh default-named service —
         # otherwise a non-default instance (an isolated integration test's
         # `-itest-` gateway) reads the default `bot-bottle-orch-gateway`, which