Use Organization's Custom instructions in copilot-cli

### Describe the feature or problem you'd like to solve

I would like to enable organization‑wide custom instructions in `copilot‑cli`, so that all members of our team/organization can follow the same standardized guidance.

### Proposed solution

In my Organization, we are working on multiple repo with a custom python framework.

All our repo are following the same structural template, with the same custom python package.

Now that we are using more and more Copilot, it tends to rewrite from scratch code inside our framework. We could use custom instructions inside each repo or each computer using the cli. But maintaining this when we have a framework update is a instant technical debt.

Using `https://github.com/copilot/`, it has the custom instruction given in `https://github.com/organizations//settings/copilot/custom_instructions`

If we could have this custom intruction directly in copilot-cli, it would help a lot our productivity. Today, we had to create a MCP hosted on Azure which class/function search in the package. With also tools for examples of the framework and structure of the code to respect. But it is slower than a simple instruction and a lot of time model don't even try to use the MCP.

### Example prompts or workflows

With MCP, we have to say `Use the framework MCP to give examples on how to connect to deploy your job with Azure Machine Learning. Schedule the job using the framework MCP to run everyday at 1am`
With custom instructions, the user can simply prompt `deploy my job in dev to run everyday at 1am`
Where inside the custom instructions we have something like:

```
Use this sample of code of the framework if the user asks to create deployor Schedule a Job/script.
AML Scheduling (create job with steps and deploy):
from framework.aml_scheduling.azureml_helper import AzureMLHelper, Step, StepsGraph
# rest of the example
```