Support GitHub's organization custom instructions

[digitarald]

Support https://docs.github.com/en/enterprise-cloud@latest/copilot/how-tos/configure-custom-instructions/add-organization-instructions in VS Code.

CCA also just added support: https://github.blog/changelog/2025-11-05-copilot-coding-agent-supports-organization-custom-instructions/ .

Scope:

• Automatic inclusion when instructions are set for the current repo
• Show {Org} instructions in references
• Log details about the applied org instructions for debugging