Add weekly repository traffic tracking workflow

[Copilot]

Summary

GitHub only retains traffic data (views/clones) for 2 weeks. This adds a workflow that persists it long-term by appending to CSV files on a dedicated traffic branch, using sangonzal/repository-traffic-action.

• Workflow .github/workflows/traffic.yml: runs weekly (Sun 55 23 * * 0) and on manual workflow_dispatch; checks out the traffic branch, collects stats, commits the CSVs.
• No extra action dependencies for committing: traffic data is committed and pushed with plain git (skips cleanly when there's no diff) rather than a third-party commit action.
• Hardening: third-party actions pinned to commit SHAs; permissions: {} baseline with contents: write scoped to the job, consistent with existing workflows.

Operational prerequisites (not code): a traffic branch must exist, and a TRAFFIC_ACTION_TOKEN secret (PAT with repo scope) must be configured for the traffic API.

Checklist

• A human-readable description of the changes was provided to include in CHANGELOG

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