refactor: extract command dispatch logic to shared module

Move command parsing and dispatching logic from `CommandLinkManager`
and `JournalInput` into a new `command-dispatcher.ts` module. This
unifies how commands are handled whether they are typed manually or
triggered via `:cmd[]` directive clicks.

src/components/journal/index.ts

@@ -49,5 +49,7 @@ export { ComposePanel } from "./ComposePanel";
 export { JournalInput } from "./JournalInput";
 export { DynamicForm } from "./DynamicForm";
 export { useJournalCompletions, invalidateCompletions } from "./completions";
+export { dispatchCommand } from "./command-dispatcher";
+export type { DispatchContext, DispatchResult } from "./command-dispatcher";
 export { JournalContext, useJournalContext } from "./JournalContext";
 export type { JournalContextValue } from "./JournalContext";