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/utils/agent-snapshots.ts

@@ -1,4 +1,3 @@
-import type { AgentLifecycleStatus } from "@server/shared/agent-lifecycle";
 import type { AgentSnapshotPayload } from "@server/shared/messages";
 import type { AgentPermissionRequest } from "@server/server/agent/agent-sdk-types";
 
@@ -31,7 +30,7 @@ export function normalizeAgentSnapshot(snapshot: AgentSnapshotPayload, serverId:
     serverId,
     id: snapshot.id,
     provider: snapshot.provider,
-    status: snapshot.status as AgentLifecycleStatus,
+    status: snapshot.status,
     createdAt,
     updatedAt,
     lastUserMessageAt,