GPUI: Companion sidebar pane does not switch when selecting another session on Windows

[maddada]

Environment

• GPUI app
• Windows

Problem

When the sidebar companion pane is visible, selecting a different session in the sidebar does not update the session displayed in the companion pane.

This occurs in panes/views other than the Agents view where the companion side pane is shown.

Steps to reproduce

1. Open a view that displays the sidebar companion pane.
2. Select one session in the sidebar.
3. Select another session.
4. Observe that the companion pane continues showing the previous session.

Expected behavior

The companion pane should immediately switch to the newly selected session.

[maddada]

Applied fix in the working tree.

• Windows now synchronizes the visible project-editor companion from the authoritative sidebar project/session focus state.
• Synchronization converges regardless of whether focus or active-project context arrives first.
• Terminal and t3code companion retargeting validates the current runtime/pane identity and preserves the user's current native focus, including async T3 creation.
• The behavior is Windows-gated; macOS/Linux paths are unchanged.

Final static verifier: PASS.

Per request, no tests, builds, app launch, commit, or push were performed.