Using simplified Beans workflow with OpenSpec

[mipmip]

I was looking for a simple workflow of buffering idea's/prompts which need an proposal when I'm in a hyperfocyus and found out working with beans works very well. Instead of the default beans instructions, here's a simple AGENTS.md instruction which implements a simple though very effecient work flow.

When I refer to issues like specgetty-63wm checkout the task
in @.beans/specgetty-63wm-*.md

In this project we will use these tasks as epics for making openspec proposals. 

WHEN you create a proposal at a link to this task in the proposal.md.
WHEN a proposal is archived add the link to the archived proposal in the front-matter of this task like this:

`openspec-link: openspec/changes/archive/....`

You are allowed to update these statuses in the task front-matter:

- in-progress                                             
- todo
- draft
- completed
- scrapped

When making changes you are allowed to update the date/time in `updated_at` in the task front-matter 

Besides updating status and openspec-link, you are NOT ALLOWED to modify the contents of the task file.

[mipmip]

Original discussion is here: hmans/beans#190