Add receiver-side opt-out for Copilot sync

[woksin]

Summary

Target repositories can now selectively opt out of receiving specific Copilot artifacts during sync/propagation by creating a .github/.copilot-sync-receive-ignore file.

Previously, filtering was only possible on the source side (.copilot-sync-ignore), meaning all target repos received the same set of files with no way to decline. This is problematic for repositories with different tech stacks — a CLI tool repo has no use for EF Core instructions or React skills.

What's new

• .github/.copilot-sync-receive-ignore — a new file that target repos can place in their repository to exclude specific incoming Copilot artifacts. Uses the same glob pattern syntax as the existing .copilot-sync-ignore (supports *, **, ?, comments with #, optional .github/ prefix).
• Extracted shared _glob_patterns_to_regex() helper in copilot-sync-ignore-filter.sh to eliminate duplication between the source-side and receive-side filter functions.
• Receive-side filtering runs after the source-side filter and before the up-to-date SHA check, so changes to ignored files won't trigger unnecessary commits.

Example usage

A CLI repository can create .github/.copilot-sync-receive-ignore:

# We don't need frontend or EF Core instructions
instructions/csharp-efcore.md
instructions/frontend/**
skills/react-*.md

# We maintain our own hooks
hooks/**

Filtering flow

Source files
  → .copilot-sync-ignore (source controls what it sends)
  → .copilot-sync-receive-ignore (target controls what it accepts)
  → SHA comparison → blob creation → direct push to default branch

Fully backward compatible — repos without the file continue to receive all artifacts.

Added

• .copilot-sync-receive-ignore support: target repos can now opt out of specific incoming Copilot artifacts by placing this file in their .github/ directory.

Changed

• Receive-side filter (_apply_copilot_sync_receive_ignore) integrated into propagate-copilot-instructions.sh using the direct-push flow introduced in main.

[woksin]

@copilot fix merge issues

[Copilot]

@copilot fix merge issues

Resolved. The only conflict was in .github/scripts/propagate-copilot-instructions.sh — main had switched propagation from a PR-based flow to a direct push, while the branch had added the receive-side filter (section 2b). The resolution keeps the receive-side filter and adopts main's direct-push approach, dropping the stale PR-creation logic. Merge commit: 5577f66.