fix(agent-gui): open message center sessions across agents

[mt-hub8]

Summary

• carry the exact Session / Agent Target / Provider identity from Message Center
• close the embedded Message Center before routing
• reuse the existing AgentGUI launch coordinator and standalone launch handler
• activate the target Session in the current Agent window without calling the native window opener
• pass the same exact identity into the existing Workbench node launch for workspace-hosted Message Center actions

Intended behavior

In Agent mode, clicking Open session:

1. closes Message Center
2. stays in the current Agent window
3. switches to the Session and Agent Target that own the message
4. never creates a new native window

Scope

This PR intentionally does not change OS notification delivery, Electron main/preload IPC, native window creation, replacement-window fallback, show/focus policy, or daemon/session lifecycle semantics.

It reuses:

• the existing Message Center model and card components
• requestWorkspaceAgentGuiLaunch
• handleStandaloneAgentGuiLaunch
• the existing Workbench Session launch request

Validation

• pnpm check:changed -- --base upstream/main: 15/15 lanes passed
• AgentGUI focused regression: 9/9 tests passed
• Desktop standalone launch regression: 5/5 tests passed
• AgentGUI typecheck: passed
• Desktop typecheck: passed
• pre-commit formatting, lint, renderer/UI/Electron boundary, and AgentGUI degradation checks: passed

The fork's default pre-push comparison uses an origin/main that is 152 commits behind upstream, so it additionally ran unrelated repository lanes; 90/91 passed and the only failure was an unrelated services/tuttid Go test. The scoped upstream-based validation above is fully green.

Manual acceptance

• open Agent A in one workspace
• ensure Message Center contains a Session owned by Agent B
• click Open session
• verify Message Center closes
• verify the same native Agent window remains
• verify header, rail, and detail show Agent B and the exact target Session
• repeat once on macOS and once on Windows

Windows requires only the same interaction check: this PR contains no Windows-specific path, process, shell, filesystem, native window, or show/focus changes.

The PR remains Draft until real-data cross-Agent acceptance is recorded.