feat(prd-0081): reprovision CA, git-gate, and egress tokens on gateway bring-up

On a Firecracker gateway cold boot, reconcile every live agent VM against
the fresh gateway: push the new mitmproxy CA into each agent's trust store,
re-provision git-gate repos/creds from the persisted upstreams snapshot,
and restore egress tokens. Per-bottle failures are logged and skipped so
one unreachable VM does not block the rest.

New modules / changes:
- backend/firecracker/reconcile.py: attach_bottled_agents_to_gateway,
  _push_ca, _reprovision_git_gate, _guest_ip_from_config
- git_gate/provision.py: write upstreams.json after key provisioning so
  the bring-up reconcile can reconstruct the upstream table without the
  manifest
- backend/firecracker/infra.py: call attach_bottled_agents_to_gateway in
  the cold-boot branch of ensure_running()
- backend/base.py: no-op default on BottleBackend
- backend/firecracker/consolidated_launch.py: remove superseded
  _reprovision_running_bottles / _guest_ip_from_config
- orchestrator: OrchestratorCore.update_agent_secret + POST
  /bottles/<id>/secret + client.update_agent_secret (single-secret
  in-place update, the reusable primitive from the 2026-07-26 hotfix)
- tests: 15 new tests in test_firecracker_reconcile.py; updated
  test_firecracker_infra.py cold-boot cases; stale FC reprovision tests
  removed from test_backend_secret_reprovision.py

Closes #516.

bot_bottle/backend/base.py

@@ -496,6 +496,20 @@ class BottleBackend(ABC, Generic[PlanT, CleanupT]):
         (macos-container) die with a pointer — the default here."""
         die(f"backend {self.name!r} has no orchestrator control plane")
 
+    def attach_bottled_agents_to_gateway(self) -> None:
+        """Reconcile all running bottles against the current gateway.
+
+        Called when the gateway is (re)brought up (cold-boot path) to restore
+        the three gateway-dependent services for every live bottle: CA trust,
+        git-gate repos/creds, and egress tokens. Per-bottle failures must be
+        logged and skipped — one unreachable agent must not block the rest.
+
+        Default: no-op. Backends that run a gateway (Firecracker, docker,
+        macOS) override this with a backend-native implementation that reaches
+        running agents via their transport (SSH for Firecracker, exec/cp for
+        docker and macOS). PRD 0081."""
+        return
+
     @abstractmethod
     def prepare_cleanup(self) -> CleanupT:
         """Enumerate orphaned resources from previous bottles. No side