Fix GitHub workflow secret access from forked PRs

[Copilot]

The .github/workflows/pr-merge.yaml workflow was failing to access secrets.DISCORD_WEBHOOK_BETA_TESTERS when triggered by pull requests from forked repositories. This is due to GitHub's security restriction that prevents pull_request events from forks accessing repository secrets.

Changes Made

1. Changed trigger from pull_request to pull_request_target

• Enables secret access by running the workflow in the target repository context instead of the fork context
• Allows secrets.DISCORD_WEBHOOK_BETA_TESTERS to be accessible for Discord notifications

2. Added proper merge condition

• Added if: github.event.pull_request.merged == true at the job level
• Ensures Discord notifications are only sent for actually merged PRs, not just closed ones
• This condition was previously commented out but is now properly implemented

3. Removed unnecessary checkout step

• Eliminated the "Checkout code" step that was marked as dangerous in the original workflow
• Since we only need to send a Discord notification, accessing the PR code is unnecessary
• Improves security by removing potential attack vector

4. Maintained security measures

• All existing user permission checks remain intact
• Still verifies the triggering user has write permissions before proceeding
• Workflow continues to be secure against malicious actors

Security Considerations

Using pull_request_target is safe in this context because:

• We removed the dangerous checkout step
• All permission verification remains in place
• The workflow only sends notifications and doesn't execute any code from the PR

The workflow will now successfully notify Discord when PRs from forked repositories are merged while maintaining proper security controls.

Fixes #4855.

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