feat(backend): reconcile running bottles' CA on gateway bring-up (0081)

Add `attach_bottled_agents_to_gateway()` as an `@abc.abstractmethod` on
`BottleBackend` and implement the first reconcile step across all three
backends: on a gateway cold boot, replace every already-running agent's
trusted CA with the freshly-minted gateway CA. Reconciling running bottles
is a backend responsibility (only the backend can enumerate its agents and
reach them — firecracker over SSH, docker/macOS over exec/cp), so making it
an abstract method keeps the fix cross-backend by construction.

The gateway rootfs is ephemeral, so a rebuild mints a new CA that every
running bottle distrusts (SSL verification fails — #510). This reconcile is
what distributes the fresh CA, so a routine gateway rebuild now doubles as a
free CA rotation. Per-bottle steps are best-effort: one unreachable or
malformed bottle is logged and skipped, never blocking the others.

Trigger — `Gateway.connect_to_orchestrator` now returns a cold-boot bool
(True when it actually (re)brought the gateway up, False when a healthy
current gateway was left untouched); each infra `ensure_running` gates the
reconcile on it, so it fires exactly on cold boot and never on an adopt.

Git-gate re-provision and egress-token restore fold into this same reconcile
on the same trigger in follow-up PRs (#516).

Refs #516. Addresses #510.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit/test_firecracker_gateway.py

@@ -61,7 +61,10 @@ class TestFirecrackerGatewayConnect(unittest.TestCase):
         booted = infra_vm.InfraVm(guest_ip="10.243.255.3", private_key=Path("/k"))
         with patch.object(infra_vm, "boot_vm", return_value=booted) as boot, \
                 patch.object(infra_vm, "push_secret") as push:
-            gw.connect_to_orchestrator(_ORCH_URL, _TOKEN)
+            cold_booted = gw.connect_to_orchestrator(_ORCH_URL, _TOKEN)
+        # The VM is booted fresh here (cold-boot path only), so it always
+        # signals a cold boot → the caller reconciles running bottles (PRD 0081).
+        self.assertTrue(cold_booted)
         # Booted on the gateway link with the gateway role; the orchestrator's
         # guest IP (parsed off the URL) rides the cmdline as bb_orch.
         kw = boot.call_args.kwargs