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/panels/agent-panel.tsx

@@ -750,7 +750,7 @@ function ChatAgentContent({
     };
   }, []);
 
-  const isInitializing = agentId ? isInitializingFromMap !== false : false;
+  const isInitializing = agentId ? isInitializingFromMap : false;
   const isHistorySyncing = useMemo(() => {
     if (!agentId || !isInitializing) {
       return false;