Add API endpoint to set organisation-level copilot instructions

[krukowskid]

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Product Feedback

Copilot Feature Area

Copilot in GitHub

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I would love to have an option to make org level instructions visible to everyone (not only org admins) with an option to contribute via PR and track changes like in repo-level instructions.
Currently there is no documented API endpoint (there is an internal endpoint used by UI /organizations/ORG_NAME/settings/copilot/custom_instructions), nor an option to reference a file from the repository in the organisation.

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