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- backend/firecracker/consolidated_launch.py: remove superseded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

@@ -9,7 +9,7 @@ backend-specific final resolution.
 from __future__ import annotations
 
 import os
-from dataclasses import dataclass, replace
+from dataclasses import dataclass
 from typing import TYPE_CHECKING, Protocol
 
 from ..agent_provider import AgentProvisionPlan, build_agent_provision_plan, get_provider
@@ -17,7 +17,6 @@ from ..egress import EgressPlan
 from ..env import ResolvedEnv, resolve_env
 from ..git_gate import GitGate, GitGatePlan
 from ..manifest import Manifest
-from ..release_manifest import oci_image
 from ..supervisor.plan import SupervisePlan
 from ..workspace import workspace_plan
 from .resolve_common import (
@@ -113,11 +112,6 @@ class BottlePreparationPlanner:
             color=spec.color,
             provider_settings=provider_config.settings,
         )
-        if not provider_config.dockerfile:
-            image, local = oci_image(
-                f"agent_{provider_config.template}", provision.image)
-            if not local:
-                provision = replace(provision, image=image)
         provision = merge_provision_env_vars(provision)
         return PreparedBottle(
             manifest=manifest,