Remove unnecessary type assertions across codebase

Add oxlint-tsgolint and configure typescript/no-unnecessary-type-assertion
to flag redundant `!` and `as Foo` casts. Type-aware mode is left off by
default to keep `npm run lint` fast; the rule sits configured for when we
turn type-aware on intentionally. Auto-fix removed ~283 redundant casts;
two manual touch-ups: a real tsgolint false positive in split-container.tsx
and a stale ChildProcess import after a double-cast collapsed.

packages/app/src/hooks/use-keyboard-shortcuts.ts

@@ -260,7 +260,7 @@ export function useKeyboardShortcuts({
     const handleCommandCenterToggle = (event: KeyboardEvent): boolean => {
       const store = useKeyboardShortcutsStore.getState();
       if (!store.commandCenterOpen) {
-        const target = event.target instanceof Element ? (event.target as Element) : null;
+        const target = event.target instanceof Element ? event.target : null;
         const targetEl =
           target?.closest?.("textarea, input, [contenteditable='true']") ??
           (target instanceof HTMLElement ? target : null);