Adds Usage/ with ClaudeOAuthUsageClient (reads the access token Claude Code keeps fresh at ~/.claude/.credentials.json, calls the Anthropic OAuth usage endpoint, defensively parses buckets/limits), UsageState (threadsafe last-good-snapshot holder that never regresses on failure), and UsageMonitorService (BackgroundService polling on the new usage_poll_interval_seconds config, min 15s, one poll at startup, warns at most once per distinct error).