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_macos_gateway.py

@@ -46,7 +46,7 @@ class TestMacosGatewayConnect(unittest.TestCase):
 
     def test_connect_signals_cold_boot(self) -> None:
         # The container is recreated unconditionally, so connect always signals
-        # a cold boot → the caller reconciles running bottles (PRD 0083).
+        # a cold boot → the caller reconciles running bottles (PRD 0081).
         run = Mock(return_value=_proc())
         with patch(f"{_GW}.container_mod") as mod, \
                 patch(f"{_GW}.host_gateway_ca_dir", return_value=Path("/ca")):