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/components/agent-status-bar.tsx

@@ -234,7 +234,7 @@ function buildFallbackAllProviderModels(
   map.set(
     provider,
     modelOptions.map((option) => ({
-      provider: provider as AgentProvider,
+      provider: provider,
       id: option.id,
       label: option.label,
     })),