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/cli/tests/run-all.ts

@@ -153,7 +153,7 @@ async function runSingleTest(testFile: string): Promise<TestOutcome> {
 
 async function runRemainingTests(index: number): Promise<void> {
   if (index >= testFiles.length) return;
-  const testFile = testFiles[index] as string;
+  const testFile = testFiles[index];
   const outcome = await runSingleTest(testFile);
   if (outcome.status === "passed") {
     passed++;