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/terminal/terminal-output-coalescer.ts

@@ -58,7 +58,7 @@ export class TerminalOutputCoalescer {
     }
 
     const payload =
-      this.chunks.length === 1 ? this.chunks[0]! : Buffer.concat(this.chunks, this.bytes);
+      this.chunks.length === 1 ? this.chunks[0] : Buffer.concat(this.chunks, this.bytes);
     const bytes = this.bytes;
     const chars = this.chars;
     this.clearPending();

packages/server/src/terminal/worker-terminal-manager.ts

@@ -1,4 +1,4 @@
-import { fork, type ChildProcess } from "node:child_process";
+import { fork } from "node:child_process";
 import { fileURLToPath } from "node:url";
 import { randomUUID } from "node:crypto";
 import type { TerminalState } from "../shared/messages.js";
@@ -109,7 +109,7 @@ function forkTerminalWorker(): TerminalWorkerProcess {
     execArgv: resolveWorkerExecArgv(),
     serialization: "advanced",
     stdio: ["ignore", "ignore", "inherit", "ipc"],
-  }) as ChildProcess as TerminalWorkerProcess;
+  }) as TerminalWorkerProcess;
 }
 
 export function createWorkerTerminalManager(