Add Copilot instructions for PR labeling

[Copilot]

Copilot-authored PRs in this repo were not being labeled, so they were mis-triaged and miscategorized in the auto-generated changelog. This mirrors PowerShellEditorServices#2310, adapted to this repository's labels and .github/release.yml.

Changes

• .github/copilot-instructions.md: Added a "Pull Request Labels" section directing Copilot to:
  • Apply at least one Area-* label and exactly one Issue-* label (Issue-Bug / Issue-Enhancement / Issue-Performance).
  • Add OS-* labels for platform-specific changes and Ignore to exclude a PR from release notes.
  • Discover the current label set via gh label list --repo PowerShell/vscode-powershell rather than hard-coding the (drifting) list.

The Issue-* guidance is tied to the changelog mapping in .github/release.yml (Issue-Enhancement → "Enhancements & Features ✨", Issue-Bug → "Squashed Bugs 🐛"), noting that a missing Issue-* label falls through the "*" catch-all into "Other Changes 🙏".

[Copilot]

Pull request overview

This PR adds a "Pull Request Labels" section to .github/copilot-instructions.md so that Copilot-authored PRs are properly labeled for triage and changelog generation. Without this guidance, Copilot PRs lacked the required Area-* and Issue-* labels, causing them to be miscategorized in auto-generated release notes.

Changes:

• Added labeling instructions requiring at least one Area-* label and exactly one Issue-* label per PR, with guidance on OS-* and Ignore labels.
• Documented the relationship between Issue-* labels and the changelog categories defined in .github/release.yml.
• Directed Copilot to discover the current label set dynamically via gh label list rather than relying on a hard-coded list.

[Copilot]

Added Issue-Performance to .github/release.yml with its own "Performance Improvements ⚡" category (commit Add Issue-Performance category to release.yml). PRs labeled Issue-Performance will now land in that section instead of the catch-all "Other Changes 🙏".