Add extensions / plugins to mux

I am currently evaluating & looking into solutions like mux, specifically with remote support etc. One thing I realized when implementing and experimenting that hooks are insufficient for the harness but extensions / plugins are crucial. Please support extensions / plugins in mux. Examples for extensions:

* [amp](https://ampcode.com/manual/plugin-api)
* [opencode](https://opencode.ai/docs/plugins/)
* [pi-mono](https://github.com/badlogic/pi-mono/blob/main/packages/coding-agent/docs/extensions.md)

Example usecase fore extension:

* when a pr is opened, track the pr outside of the llm and **send a message to it when it changed**
* classify if the agent is asking/waiting for input of the user and then show a ui for it
* introduce new modes with different prompts etc.
* make new tools available to the agent
* load agents.md or other files from a central place

Supporting extensions will simplify customization and allow organizations to tailor mux for their need and simplify the surface area as aspects like notifications etc. can be deployed as extension instead of be part of the core.