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/firecracker/infra_launch.py

@@ -90,7 +90,7 @@ def persist_env_var_secret(private_key: Path, guest_ip: str, secret: str) -> Non
 def attach_ca_to_agent_vm(run_dir: Path, ca_pem: str) -> None:
     """(Re)attach one running agent VM (identified by its `run_dir`) to the
     current gateway: replace its trusted gateway CA with `ca_pem` and rebuild
-    its trust store over SSH (PRD 0083). Unconditional install — there is one
+    its trust store over SSH (PRD 0081). Unconditional install — there is one
     gateway, so no fingerprint match is needed. Bare-pipe input keeps the cert
     off argv.