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).

tests/unit/test_docker_infra.py

@@ -70,7 +70,7 @@ class TestDockerInfraService(unittest.TestCase):
 
     def test_cold_boot_reconciles_running_bottles(self) -> None:
         # A (re)created gateway (connect returns True) triggers the bring-up
-        # reconcile so running bottles re-trust the fresh gateway (PRD 0083).
+        # reconcile so running bottles re-trust the fresh gateway (PRD 0081).
         self.gw.connect_to_orchestrator.return_value = True
         self.svc.ensure_running()
         self.attach.assert_called_once_with()