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- backend/firecracker/consolidated_launch.py: remove superseded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/macos_container/launch.py

@@ -38,7 +38,6 @@ import subprocess
 from contextlib import ExitStack, contextmanager
 from typing import Callable, Generator
 
-from ...agent_provider import runtime_for
 from ...bottle_state import (
     egress_state_dir,
     git_gate_state_dir,
@@ -94,14 +93,7 @@ def _agent_image(plan: MacosContainerBottlePlan) -> str:
             )
         info(f"using cached agent image {plan.image!r}")
         return plan.image
-    if "@sha256:" in plan.image:
-        container_mod.pull_image(plan.image)
-        container_mod.verify_agent_image(
-            plan.image, runtime_for(plan.agent_provider_template).smoke_test,
-        )
-        return plan.image
-    container_mod.build_image(
-        plan.image, str(resources.build_root()), dockerfile=plan.dockerfile_path)
+    container_mod.build_image(plan.image, str(resources.build_root()), dockerfile=plan.dockerfile_path)
     return plan.image