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/directory-suggestions.ts

@@ -889,7 +889,7 @@ function pruneDirectoryListCache(): void {
   }
 
   while (directoryListCache.size > DIRECTORY_LIST_CACHE_MAX_ENTRIES) {
-    const oldestKey = directoryListCache.keys().next().value as string | undefined;
+    const oldestKey = directoryListCache.keys().next().value;
     if (!oldestKey) {
       return;
     }
@@ -910,7 +910,7 @@ function pruneWorkspaceEntryListCache(): void {
   }
 
   while (workspaceEntryListCache.size > DIRECTORY_LIST_CACHE_MAX_ENTRIES) {
-    const oldestKey = workspaceEntryListCache.keys().next().value as string | undefined;
+    const oldestKey = workspaceEntryListCache.keys().next().value;
     if (!oldestKey) {
       return;
     }