Add workflow to enforce main branch target for external contributor PRs

[Copilot]

External contributors occasionally target release/* or feature/* branches instead of main, requiring manual intervention to redirect them.

Changes

• New workflow: .github/workflows/validate-pr-target-branch.yml
  • Triggers on PR open/reopen/edit (when base branch changes)
  • Checks author permissions via GitHub API
  • Auto-closes PRs from external contributors (no write access) targeting non-main branches
  • Posts explanatory comment with link to contribution guidelines

Implementation Details

• Uses pull_request_target for secure execution in base repo context
• Differentiates between internal (write/admin) and external (read/none) contributors
• Skips runs when PR edited but base unchanged (performance optimization)
• Dynamic repo context for portability
• Graceful error handling (no action if permission check fails)

Behavior

Scenario	Action
External contributor → main	✅ Allow
Internal contributor → any branch	✅ Allow
External contributor → release/9.0	❌ Close + comment

Original prompt

Create a Github action that closes PRs that were submitted by an external contributer (no push permission) if they target any branch other than main.
It should also post a comment on the PR telling them to resubmit it with the correct target branch.
If not possible to automate in this way, suggest alternatives.

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[Copilot]

Pull request overview

This PR adds automation to enforce that external contributors target the main branch for their pull requests, addressing a recurring issue where external PRs mistakenly target release or feature branches.

Changes:

• Adds a new GitHub Actions workflow that validates PR target branches based on contributor permissions
• Automatically closes PRs from external contributors that target non-main branches
• Posts an explanatory comment directing contributors to the contribution guidelines