docs(prd): reconcile via a backend-ABC method across all backends (0081 review)

Address review: target all backends, not firecracker-only. Make the reconcile
an @abc.abstractmethod `attach_bottled_agents_to_gateway()` on BottleBackend so
every backend implements it (cross-backend by construction) and the host calls
it on gateway bring-up. Drop the docker/macOS non-goal; add the open question of
retiring docker's now-redundant CA bind-mount.

Refs #516

docs/prds/0081-reprovision-gateway-state-on-bringup.md

@@ -49,29 +49,40 @@ destroy (the original #450 failure mode).
   running agents by the same reconcile.
 - Per-bottle failures are tolerated: one unreachable or malformed bottle does not
   block the others or the gateway coming up.
+- **All backends** (Firecracker, docker, macOS) reconcile through the *same*
+  contract — an abstract method on the backend base class, so a new backend
+  cannot forget to implement it and none drifts onto a bespoke mechanism.
 
 ## Non-goals
 
 - Deliberate mid-session CA rotation *without* a gateway restart — `rotate_ca`
   stays for that operator action.
-- The docker / macOS backends. This lands the reconcile for Firecracker (the
-  backend that boots a fresh gateway rootfs each time); docker keeps its existing
-  host-bind-mounted CA until the same seam is extended there (follow-up). The
-  unification target is *per-service*, not *per-backend*.
 - Changing source-IP attribution, `/resolve`, or the plane split (#469).
 
 ## Design
 
-**Trigger — the gateway startup flow.** `FirecrackerGateway.connect_to_orchestrator`
-is the cold-boot path: `FirecrackerInfraService.ensure_running` calls it only when
-it boots a fresh pair, and skips it when it adopts a healthy, current gateway
-(state intact). So reconcile fires exactly when the gateway was (re)booted —
-including orchestrator restarts, since the pair boots together — and never on an
-adopt. There is no bare-restart path that bypasses `ensure_running`.
+**The contract — `attach_bottled_agents_to_gateway()` on the backend ABC.**
+Reconciling running bottles against the current gateway is a backend
+responsibility (only the backend can enumerate its agents and reach them —
+firecracker over SSH, docker/macOS over `exec`/`cp`), so it is an
+`@abc.abstractmethod` on `BottleBackend` (`backend/base.py`). Every backend
+implements it; the host calls it whenever the gateway is (re)brought up. This is
+what makes the fix cross-backend by construction rather than a per-backend
+follow-up. It reprovisions **all** registered bottles' gateway-dependent state:
+CA, git-gate, and egress tokens.
 
-**`reconcile_running_bottles(gateway, orchestrator_url)`** (new
-`firecracker/reconcile.py`), called at the end of `connect_to_orchestrator` once
-the gateway VM is up and its CA is available:
+**Trigger — the gateway bring-up path.** The host calls
+`attach_bottled_agents_to_gateway()` only when the gateway was actually
+(re)brought up — the cold-boot branch of the infra bring-up (e.g.
+`FirecrackerInfraService.ensure_running` after it boots a fresh pair), never on an
+adopt of a healthy, current gateway (state intact). So it fires exactly when the
+gateway was (re)booted — including orchestrator restarts, since the pair boots
+together — and there is no bare-restart path that bypasses bring-up.
+
+**Per-backend implementation.** Each backend's `attach_bottled_agents_to_gateway`
+enumerates its live bottles and, for each, reconciles the three services against
+the current gateway. The firecracker implementation, once the gateway VM is up
+and its CA is available:
 
 1. Map each live bottle's guest IP → `bottle_id` from the orchestrator registry
    (`list_bottles`).
@@ -100,6 +111,7 @@ as a CA rotation with zero extra machinery.
 
 ## Open questions
 
-- Docker / macOS parity: extend the same reconcile seam (docker `exec` instead of
-  SSH) and drop docker's CA bind-mount, or leave docker on persistence? Deferred
-  to a follow-up; not blocking the Firecracker fix.
+- Docker's existing host-bind-mounted CA (`host_gateway_ca_dir`): once docker's
+  `attach_bottled_agents_to_gateway` pushes the CA to running agents on bring-up,
+  the bind-mount is redundant — drop it (so docker rotates like firecracker) or
+  keep it as belt-and-suspenders? Leaning drop, for one behaviour across backends.