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/server/src/utils/checkout-git.ts

@@ -1478,11 +1478,11 @@ function parseCheckoutShortstat(text: string): CheckoutShortstat | null {
   let deletions = 0;
   const addMatch = trimmed.match(/(\d+)\s+insertion/);
   if (addMatch) {
-    additions = Number.parseInt(addMatch[1]!, 10);
+    additions = Number.parseInt(addMatch[1], 10);
   }
   const delMatch = trimmed.match(/(\d+)\s+deletion/);
   if (delMatch) {
-    deletions = Number.parseInt(delMatch[1]!, 10);
+    deletions = Number.parseInt(delMatch[1], 10);
   }
 
   if (additions === 0 && deletions === 0) {