feat: add OTEL metrics for per-skill usage tracking

feat: add OTEL metrics for per-skill usage tracking PostToolUse hook on Skill sends a protobuf-encoded OTLP metric (skill.invocation.count) with skill.name, user.name, and user.email attributes to the configured OTEL endpoint. Pure bash — no Python or pip dependencies. Co-Authored-By: Claude Opus 4.6 (1M context)