Create an EventManager class in context transport primitives

[lukemartinlogan]

@AGENTS.md

Create an EventManager class in context transport primitives. I want to migrate all OS-dependent code to the context transport primitives as much as possible. This task focuses on abstracting signals and epoll.

struct Event {
  int fd_;  // Linux
};

class EventManager {
  // Add a custom event like a file descriptor for waiting for I/O competions, etc.
  void AddEvent(const Event &event);
  // Signal process awake
  void Signal(int pid);
  // Poll until event or re-awaken after 
  void PollFor(u32 usec);
}

This is essentially a wrapper around epoll.
On Linux, the EventManager allows for custom events to be registered.
By default, there will be an event for "re-awakening". This is equivalent to our signaling code with SIG_USR1 we had in the IpcManager. That should be moved here.
This class should automatically create an epoll file descriptor and a signal fd. This code should be removed from Worker and placed here. Worker should use this class instead. Same for the IpcManager.