Create a global context

Problem/Motivation

Watching the Driesnote from Vienna 2025 https://www.youtube.com/live/WNEyg3UKTVQ?si=m9tP0ULg-G4dzyT3&t=3525, it focused on simply adding a context that "all the AI agents in Drupal have access to this context" and context being updated by "site builders and marketers"

I think the way the contexts are setup now in the module achievin this would be a fairly complicated but it could be made much easier by adding a single "global" context in addition to the current contexts.

The problems

1. Achieving "all the AI agents in Drupal have access to this context" is difficult. For a site just a few agents this may be simple but you have to go to admin/config/ai/ai-context/pools and edit every agent to assign the context to each one

   Also every time a new module is installed or an existing module is updated it may provide a new agent. In this case the site builder would have to add the context to every new agent manually(this doesn't seem like something a marketer should be expected to do)

2. Marketers would expected know how to make a context available to every agent. While we expect a site builder to be able to figure this out or read the docs it seems overly complicated for someone who does know how agents work to figure out.
4. Dries compare the contexts to a "claude.md" file. Although you make multiple claude.md files in different directories the simplest set up is to put 1 in the base of your project and it is always in affect. Nothing more to set up , nothing else to think about. Conceptually the contexts are setup now are not as simple

Steps to reproduce

Proposed resolution

1. Leave the existing contexts behavior the way it currently is
2. Add a new single global context, this would always be included and would not require any other setup
3. Add a new "edit global context" permission. This would allow editing only the new global context

This would allow a marketer to be assigned the ""edit global context" permission as have single text are to edit which affect all AI operations on their site

Remaining tasks

User interface changes

New Global Context form and menu item

API changes

Data model changes