refactor(adapters): extract shared usage helpers

[samzong]

What's changed?

• Add src/adapters/usage.rs with shared RawUsageEvent::observed / derived builders and usage_count helper
• Refactor Claude Code, Codex, Cursor, Gemini, Grok, OpenCode, and Pi adapters to use struct-update syntax instead of repeating default token fields
• Move duplicated usage_count from Pi into the shared module; reuse it in Cursor and Gemini

Why

• Adapters were repeating the same RawUsageEvent scaffolding (event keys, zero defaults, token source, parser version) across seven files
• Centralizing builders and token counting reduces drift and makes future usage parsing changes smaller and safer