Add weaver and proto repositories to the PR dashboard workflow.

[jsuereth]

Set up bare minimum so we have dashboards for maintainers.

Particularly interested in proto, but expanding for all three weaver repositories too.

[Copilot]

Pull request overview

This PR opts three OpenTelemetry Weaver repositories (weaver, opentelemetry-weaver-examples, opentelemetry-weaver-packages) and opentelemetry-proto into the shared PR-triage dashboard by adding entries to repositories.json. The dashboard workflow reads this file to build a matrix and generate per-repository PR dashboards, so these additions give maintainers of those repos a dashboard. I verified all four repository names correspond to real open-telemetry repositories, and the new entries conform to the schema documented in pull-request-dashboard/README.md (name, approver_teams, optional required_approvals, slack_channel, slack_user_mapping), with alphabetical ordering preserved.

Changes:

• Added opentelemetry-proto (2 required approvals, proto-approvers).
• Added the three Weaver repos (opentelemetry-weaver-examples, opentelemetry-weaver-packages, weaver) with their approver teams, defaulting to 1 required approval.
• Slack fields left empty, which skips Slack processing for these repos (consistent with several existing entries).

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