Replace the boolean "skip first render" ref with a ref that stores the
previously-seen tick value. The effect now compares against the stored
value and only fires when it differs, which makes the dependency
naturally used (removes the `void slideoutOpenedTick;` acknowledgement)
and self-documents the intent of the guard.
Behavior is unchanged — both forms preserve the one-shot-per-event
semantics of the prior window-event implementation. The JSDoc on
`slideoutOpenedTickAtom` is updated to describe the new pattern.
Replace the `SLIDEOUT_PANEL_OPENED_EVENT` window event with a
`slideoutOpenedTickAtom` jotai atom. The dispatcher in
`SidebarSlideOutPanel` now bumps the tick via `useSetAtom`, and the
listener in `Thread` reads it via `useAtomValue` and reacts on change
behind a ref guard that skips the initial render — preserving the
one-shot-per-open semantics of the previous event.
This removes the implicit cross-module string contract, makes the
signal traceable through React DevTools / jotai inspector, and lets
TypeScript catch typos that the string-based event API silently
swallowed.
