[codex] Sync sidebar cursor with active target

[samleeney]

Summary

Fix the sidebar cursor so it follows the active tmux target on startup and whenever the active client session/window/pane changes.

Root cause

The sidebar kept its keyboard cursor at the first selectable session row unless the user moved it. The active tmux target was rendered separately with the current-row styling, so the UI could show an unrelated row with the grey cursor and arrow while the real active row was also highlighted.

Changes

• Add a small sidebar selection helper that finds the best selectable row for the active target.
• Prefer exact pane matches, then active window rows, then the active session/worktree row.
• Sync the sidebar cursor after collecting a changed active client target.
• Add regression coverage for session, pane, window fallback, inbox duplicate, and worktree selection cases.

Validation

• bash -n scripts/sidebar.sh scripts/lib/sidebar-selection.sh tests/sidebar-active-selection.sh
• for t in tests/*.sh; do bash "$t"; done