[EM] Automate verified sequential gateway restarts

[astandrik]

Goal

Make Gateway deployment verify both the installed artifact and the code actually running, with controlled activation across hosts.

Work

• Review the applicable deployment workflow and its package-install hooks.
• Activate hosts sequentially with readiness and stability checks.
• Stop progression on a failed check and report actionable diagnostics.
• Make repeated execution and rollback predictable.

Done when

• A multi-host rehearsal verifies one-host-at-a-time activation.
• Installed and running artifact identities are checked.
• HTTP and MCP readiness, client recovery and process stability are covered.
• Unhealthy hosts stop further activation.
• Repeat execution and rollback are tested and documented.

Tracked in the UI backlog; implementation belongs to deployment tooling. Production execution is separate.