Document fine-grained PAT limitation for organization repositories

[Copilot]

GitHub platform limitation: organization-owned fine-grained PATs cannot have "Copilot Requests" permission, blocking CI/CD automation for org repos.

Changes

• Token Setup section: Added "Organization Repositories" subsection documenting the two-token workaround pattern
• Troubleshooting section: New entry explaining the limitation with root cause and solution
• action.yml: Updated copilot-token and repo-token descriptions to clarify org repo usage
• File encoding: Converted README.md from UTF-16 to UTF-8

Workaround Pattern

- uses: austenstone/copilot-cli@v2
  with:
    copilot-token: ${{ secrets.COPILOT_TOKEN }}  # User PAT with "Copilot Requests"
    repo-token: ${{ secrets.GITHUB_TOKEN }}      # Org token for repo operations

The copilot-token authenticates with Copilot API using a user-owned PAT, while repo-token handles repository operations with org credentials.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Question] Copilot Requests permission unavailable for organization-owned repos, fine-grained PATs blocks CI/CD automation</issue_title>
<issue_description>### Question

Summary

When using GitHub Copilot CLI (@github/copilot) in GitHub Actions for organization repositories, authentication fails because fine-grained PATs cannot have both "Copilot Requests" permission AND organization repository access simultaneously.

Problem

Fine-grained Personal Access Tokens have a fundamental limitation:

Token Resource Owner	Copilot Requests Permission	Org Repository Access
Personal Account	✅ Available	❌ Limited
Organization	❌ Not Available	✅ Full Access

This forces users to choose between:

1. A user-owned token with Copilot access but limited org repo visibility
2. An org-owned token with repo access but no Copilot authentication

Use Case

We are trying to automate release notes generation using Copilot CLI in GitHub Actions:

- name: Generate release notes with Copilot
  env:
    COPILOT_GITHUB_TOKEN: ${{ secrets.COPILOT_TOKEN }}
  run: |
    copilot -p "Generate release notes for changes in the last 7 days across all repos in 'our-org'" --allow-all-tools < /dev/null

Error

When using an org-owned fine-grained PAT (with repo access but no Copilot Requests permission):

The organization 'org-name' either doesn't exist or isn't accessible.

When using a user-owned fine-grained PAT (with Copilot Requests but limited org access):

• Cannot access organization repositories

Expected Behavior

Organization-owned fine-grained PATs should have the "Copilot Requests" permission available, allowing enterprises to:

• Use Copilot CLI in CI/CD pipelines
• Access organization repositories with proper scoping</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Question] Copilot Requests permission unavailable for organization-owned repos, fine-grained PATs blocks CI/CD automation #64

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