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/client/daemon-client.ts

@@ -1124,7 +1124,7 @@ export class DaemonClient {
       return this.subscribe(arg1);
     }
 
-    const type = arg1 as SessionOutboundMessage["type"];
+    const type = arg1;
     const handler = arg2 as (message: SessionOutboundMessage) => void;
 
     if (!this.messageHandlers.has(type)) {