Enhance clone-stats workflow to persist history and improve error handling

[Sendipad]

Motivation

• Ensure clone metrics survive the GitHub traffic API 14-day window by maintaining a persistent history file.
• Make the workflow reliable and actionable by requiring a TRAFFIC_TOKEN (classic PAT with repo scope) and surfacing clear error messages for common API failures.
• Update the public README badge to report all-time clones using the persisted cumulative total.

Description

• Updated .github/workflows/clone-stats.yml to restore an existing stats/clone_history.json, merge new daily snapshots, compute a total_count, and write stats/clone_history.json and stats/clones.json on each run.
• Reworked the traffic fetch to require TRAFFIC_TOKEN, capture HTTP status codes from curl, and fail with explicit messages for 401 and 403 responses and other non-2xx responses.
• Added git add stats/clone_history.json so the history file is committed and pushed to the stats branch.
• Updated README.md to change the clones badge to read total_count (all-time) and updated stats/README.md to document the new total_count, the stats/clone_history.json artifact, and common troubleshooting for 401/403 errors.

Testing

• No automated tests were added or run for these workflow and documentation changes.

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Codex Task