fix(runtime-quickjs): offload QuickJS executions to worker threads in selfhost to prevent event loop blocking

[Adityakk9031]

Fixes #1616

Summary

Self-host executions in @executor-js/runtime-quickjs previously ran QuickJS synchronously on the Node.js/Bun server's main thread. When a CPU-bound execution was run (e.g. while (true) {}), it monopolized the main event loop until hitting timeoutMs (up to 5 minutes by default), starving health probes, OAuth sessions, web UI, and concurrent MCP requests.

This PR introduces optional worker thread execution to @executor-js/runtime-quickjs and enables it for self-hosted instances:

1. Worker Thread Execution (worker: true): Added worker?: boolean option to QuickJsExecutorOptions. When enabled, makeQuickJsExecutor delegates execution to a dedicated node:worker_threads.Worker running packages/kernel/runtime-quickjs/src/worker.ts.
2. Bidirectional IPC: Tool invocations inside the QuickJS worker seamlessly bridge back to the parent thread's toolInvoker and yield results/errors back to the worker context.
3. Cancellation & Preemption: Interruption or timeout terminates the worker thread cleanly without hanging or stalling the host server's main event loop.
4. Self-Host Enabled: SelfHostCodeExecutorProvider in apps/host-selfhost/src/execution.ts sets worker: true.

Verification

• Unit tests added to packages/kernel/runtime-quickjs/src/index.test.ts covering plain executions, bidirectional tool dispatches, tool error propagation, and CPU-bound loop preemption in worker threads.
• Vitest tests:
  • packages/kernel/runtime-quickjs: 32/32 tests passed (including all worker thread suites).
  • apps/host-selfhost: 137/137 tests across 24 test files passed.
• Gates:
  • bun run typecheck: 44/44 successful.
  • bun run lint: 0 warnings, 0 errors.
  • bun run format:check: All modified files formatted and verified.