Score extension pull events for extension authors

Summary

When someone pulls an extension, the extension's author should receive score points through the telemetry pipeline. This rewards authors for building extensions that others find useful.

Motivation

Extension pull stats are already tracked (extension_pull_repository), but they only feed into refresh-extension-score.ts at score-refresh time as a popularity signal. Scoring pulls as telemetry events gives authors real-time credit as their extensions gain traction, and integrates with the same pipeline as other scoring events.

Pull events should be worth less than publish events (#228) — publishing is the higher-effort contribution, while pulls are a passive reward for having published something useful.

Design sketch

• Event source: Pull recording in the extension download path emits an event crediting the extension author
• Event shape: { event: "extension_pull", distinct_id: "...", properties: { authorUsername, extensionName, version, pulledBy? } }
• Scoring: Small fixed points per pull, significantly less than a publish event. Could have diminishing returns (e.g., first 100 pulls worth more than the next 100).
• Deduplication: Same puller + extension + version should only score once (prevent refresh-spam), or score uncapped if anonymous pulls are included

Open questions

• Points per pull vs. diminishing returns curve?
• Score anonymous pulls the same as authenticated ones?
• Should self-pulls (author pulling their own extension) be excluded?
• Relationship to existing refresh-extension-score.ts — replace it, supplement it, or keep both?
• Relationship to #227 (award events) and #228 (extension publish scoring)