feat: rebuild reactivity state from store after replay

Add `rebuildReactivityFromStore` to synchronize local reactivity
state with the hydrated variable store. This ensures tag activations
are correctly tracked following a reducer replay.

Also remove the unused `invalidateCompletions` function.

src/components/journal/completions.ts

@@ -209,12 +209,6 @@ export function ensureCompletions(): Promise<void> {
   return _initPromise;
 }
 
-/** Force a re-fetch on the next page load. For runtime, call before invalidate triggers. */
-let _invalidated = false;
-export function invalidateCompletions(): void {
-  _invalidated = true;
-}
-
 /**
  * Reactive hooks for the journal input.
  * Returns the current completions state + a convenience `data` extractor.