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/e2e/terminal-keystroke-stress.spec.ts

@@ -278,7 +278,7 @@ async function measureDaemonBurstEcho(
       timeoutMs: STRESS_TIMEOUT_MS,
     });
 
-    const latencies = sendTimes.map((sentAt, index) => outputTimesByByte[index]! - sentAt);
+    const latencies = sendTimes.map((sentAt, index) => outputTimesByByte[index] - sentAt);
     return {
       inputTextLength: inputText.length,
       inputFrameCount: sendTimes.length,