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).

docs/decisions/0006-shared-behavior-in-base-backend.md

@@ -15,7 +15,7 @@ macOS-container). Cross-backend operations can be expressed two ways:
    concrete method and each backend overrides small **primitives** it calls.
 
 Form 1 gives each backend total freedom, which is exactly the problem: the three
-implementations drift. The bring-up reconcile (PRD 0083) first shipped as form 1
+implementations drift. The bring-up reconcile (PRD 0081) first shipped as form 1
 and immediately diverged — each backend re-implemented "gather CA → list running
 bottles → push to each", and one backend's copy silently skipped failures while
 another's aborted, an inconsistency caught only in review (#519). The behaviour
@@ -37,7 +37,7 @@ method that:
 - delegates only the irreducibly backend-specific steps to `@abstractmethod`
   primitives with narrow, well-typed signatures.
 
-The first application is `attach_bottled_agents_to_gateway()` (PRD 0083): the
+The first application is `attach_bottled_agents_to_gateway()` (PRD 0081): the
 base gathers resources once, attempts every running bottle, and raises an
 aggregate `InfraLaunchError` if any failed; backends supply only
 `_gateway_attach_resources()`, `_running_bottles()`, and
@@ -60,6 +60,6 @@ operations whose *shape* should be uniform, not about banning abstract methods.
 
 ## Links
 
-- PRD 0083 (`docs/prds/0083-reprovision-gateway-state-on-bringup.md`).
+- PRD 0081 (`docs/prds/0081-reprovision-gateway-state-on-bringup.md`).
 - Review that prompted this: PR #519.
 - `bot_bottle/backend/base.py` (`BottleBackend.attach_bottled_agents_to_gateway`).