Add multi-host boot consistency smoke test for installed agents

## Summary
Add a smoke/integration scenario that boots the shell, agentServer, and web API host paths against one fresh instance directory and verifies they observe the same installed-agent state.

## Background
The per-host wiring is covered by build/type-check and unit tests around the shared installed-agent records, but no single test boots all host entry points against the same `agents.json` / install-source config and verifies consistent visibility.

## Suggested Coverage
- create a fresh instance directory with installed-agent state
- boot shell, agentServer, and API host paths against that instance directory
- verify all hosts see the same installed agents and source-derived state
- include restart/reload behavior if the smoke harness can do so cheaply

## Source
Deferred from PR #2611 review-log cleanup.

Start with the smoke/integration harness and the shell, agentServer, and web API host entry points. Create a fresh instance directory containing agents.json and the install-source configuration, then boot all three paths against it. Done means every host reports the same installed agents and source-derived state, with restart/reload coverage if the harness supports it cheaply.

Written by the indexing model from the issue text.