Clicking a bot row moved the layout interaction tracker to the sidebar
group, so $focusedStoredSessionId fell back to the primary selection —
which is null in Bot Mode, because bot chats open as tiles and never set
$selectedStoredSessionId. The Bots plugin reads that null 'focused'
edge as 'the chat lost the center', releases its open claim, and the
Bots home re-asserts over the still-visible chat: the UI jumps to the
list instead of staying in the chat.
$focusedStoredSessionId now answers from the main zone's active tile in
Bot Mode before falling back to the selection, so a chrome-sidebar
click no longer fabricates a null edge; a genuinely closed chat (no
tile in main) still surfaces null and the home returns as before.
Regression tests cover the sidebar-click case (red before the fix),
plus guards for the closed-chat and sessions-mode derivations.
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