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- backend/firecracker/consolidated_launch.py: remove superseded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

@@ -20,7 +20,6 @@ import subprocess
 import sys
 from pathlib import Path
 
-from ... import invocation
 from ... import resources
 from . import netpool
 from . import util
@@ -180,11 +179,9 @@ def _setup_systemd() -> None:
         f"sudo systemctl daemon-reload\n"
         f"sudo systemctl enable --now {netpool.SYSTEMD_UNIT}\n"
     )
-    # Absolute path, not `sudo bot-bottle`: sudo's secure_path drops
-    # ~/.local/bin, where both pipx and install.sh put the entry point.
     sys.stderr.write(
-        f"\n(Or re-run this as root to install it directly:\n"
-        f"    {invocation.sudo_command('backend', 'setup', '--backend=firecracker')})\n"
+        f"\n(Or re-run this as root to install it directly: "
+        f"sudo bot-bottle backend setup --backend=firecracker)\n"
     )