docs: document GitHub Copilot coding agent prerequisites and repository visibility requirements

[Copilot]

Users couldn't find a specific repository in the Copilot coding agent's "Select repository" dropdown at github.com/copilot/agents, and questioned whether the existing copilot-instructions.md constitutes a valid setup.

Why I did it

The copilot-instructions.md lacked any documentation on how the Copilot coding agent works and what conditions must be met for a repository to appear in the agent's repository selector. This left users without actionable guidance when a repo was missing from the dropdown.

How I did it

Added a ## GitHub Copilot Coding Agent section to .github/copilot-instructions.md covering:

• Confirmation that this file is a valid agent configuration (agent reads it for project context when assigned to an issue, then opens a PR)

• Enumerated prerequisites for a repository to appear in the "Select repository" dropdown:

  1. Copilot Business/Enterprise enabled for the owning org/account
  2. Write access to the repository
  3. GitHub Actions enabled on the repository
  4. Repository is not archived
  5. Authenticated with the account that has access

• A one-line troubleshooting note directing users to check the above settings and refresh their session

How to verify it

Read .github/copilot-instructions.md — the new section appears after ## Gotchas and accurately describes the agent workflow and dropdown prerequisites.

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111

Description for the changelog

docs: add GitHub Copilot coding agent setup section to copilot-instructions.md, documenting repository dropdown prerequisites and troubleshooting guidance

Link to config_db schema for YANG module changes

N/A

A picture of a cute animal (not mandatory but encouraged)

Original prompt

Is this a working setup? Why I am not able to see the science-bookshelf repository in the Select repository drop down menu ?

The user has attached the following file paths as relevant context:

• .github\copilot-instructions.md

Created from VS Code.

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