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- backend/firecracker/consolidated_launch.py: remove superseded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

@@ -228,13 +228,8 @@ def build_or_load_agent_base(plan: FirecrackerBottlePlan) -> Path:
         info(f"resuming from committed rootfs {committed_tar}")
         return util.build_committed_rootfs_dir(committed_tar)
     dockerfile = Path(plan.dockerfile_path)
-    prebuilt = "@sha256:" in plan.image
     if plan.spec.image_policy == "cached":
-        cached = (
-            image_builder.cached_agent_image_rootfs_dir(plan.image)
-            if prebuilt
-            else image_builder.cached_agent_rootfs_dir(dockerfile)
-        )
+        cached = image_builder.cached_agent_rootfs_dir(dockerfile)
         if cached is None:
             die(
                 f"cached agent rootfs for {plan.image!r} not found; "
@@ -242,11 +237,6 @@ def build_or_load_agent_base(plan: FirecrackerBottlePlan) -> Path:
             )
         info(f"using cached agent rootfs {cached.name}")
         return cached
-    if prebuilt:
-        return image_builder.acquire_agent_image_rootfs_dir(
-            plan.image,
-            smoke_test=runtime_for(plan.agent_provider_template).smoke_test,
-        )
     return image_builder.build_agent_rootfs_dir(
         dockerfile,
         image_tag=plan.image,
@@ -263,11 +253,7 @@ def stale_checks(plan: FirecrackerBottlePlan) -> None:
     if committed and committed_tar.is_file():
         check_stale_path(f"agent rootfs {committed_tar}", committed_tar)
         return
-    cached = (
-        image_builder.cached_agent_image_rootfs_dir(plan.image)
-        if "@sha256:" in plan.image
-        else image_builder.cached_agent_rootfs_dir(Path(plan.dockerfile_path))
-    )
+    cached = image_builder.cached_agent_rootfs_dir(Path(plan.dockerfile_path))
     if cached is not None:
         check_stale_path(f"agent rootfs {cached}", cached / ".bb-ready")