Score sign_up events in the telemetry pipeline

Summary

Account creation should grant a baseline score through the telemetry pipeline, giving new users an immediate non-zero score upon signing up.

Motivation

New users start at score 0 and stay there until CLI telemetry events are processed. A sign-up scoring event would give every new operative an immediate presence on the leaderboard and a starting score that acknowledges joining the swamp.

Design sketch

• Event source: BetterAuth signup hook (or the existing Mixpanel sign_up tracking in middleware) emits a telemetry event
• Event shape: { event: "sign_up", distinct_id: "...", properties: { username } }
• Scoring: Fixed point value added as a one-time contribution — not run through the activity scoring algorithm
• Deduplication: One sign-up per username, idempotent on reprocessing

Open questions

• How many points? Enough to differentiate from zero but not enough to affect tier placement
• Emit from BetterAuth hook vs. reuse existing middleware tracking?
• Relationship to #227 (award events) — could sign-up just be a specific kind of award?