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/index.ts

@@ -48,7 +48,7 @@ export { StreamMessageCard } from "./StreamMessage";
 export { ComposePanel } from "./ComposePanel";
 export { JournalInput } from "./JournalInput";
 export { DynamicForm } from "./DynamicForm";
-export { useJournalCompletions, invalidateCompletions } from "./completions";
+export { useJournalCompletions } from "./completions";
 export { dispatchCommand } from "./command-dispatcher";
 export type { DispatchContext, DispatchResult } from "./command-dispatcher";
 export { parseInput } from "./command-parser";