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-runtime-metrics.ts

@@ -138,7 +138,7 @@ export class DaemonClientRuntimeMetrics {
 
   private pruneBuckets(now: number): void {
     const cutoff = now - this.windowMs;
-    while (this.buckets.length > 0 && this.buckets[0]!.endedAt < cutoff) {
+    while (this.buckets.length > 0 && this.buckets[0].endedAt < cutoff) {
       this.buckets.shift();
     }
   }

packages/server/src/client/daemon-client-websocket-transport.ts

@@ -128,7 +128,7 @@ export function bindWsHandler(
       }
     };
   }
-  const prop = `on${event}` as "onopen" | "onclose" | "onerror" | "onmessage";
+  const prop = `on${event}`;
   const wsRecord = ws as unknown as Record<string, unknown>;
   const previous = wsRecord[prop];
   wsRecord[prop] = handler;

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)) {