[Feature Request] `/stats` Local Usage Statistics Command

[myk1yt]

Summary

Add a /stats slash command that displays local token usage statistics for LLM API calls, similar to Claude Code's /stats feature.

Demo Video

▶️ YouTube에서 보기

Problem

Users have no way to track their actual token consumption across different models, providers, and sessions. Current UI shows per-task token counts but doesn't provide aggregate statistics over time.

Proposed Solution

A /stats command that opens a dedicated statistics panel showing:

Core Features

• Time ranges: Today, 7 days, 30 days, All time
• Token breakdown: Input, Output, Cache Read, Cache Write, Reasoning tokens
• Grouping: By model, provider, Zoo Code mode, status (completed/failed/cancelled)
• Time-axis aggregation: Daily, weekly, monthly views
• Daily heatmap: Visual representation of usage over time
• Export: JSON and CSV formats
• Reset: With confirmation dialog

Technical Design

• Append-only event log: Each API call records a UsageEvent as NDJSON
• Local storage only: Data stored in globalStorage/usage-stats/
• No sensitive data: API keys, prompts, and responses are never stored
• Single instrumentation point: Task.ts terminal finalize (not per-chunk)
• Idempotent recording: Prevents duplicate events from retries

Safety

• Data is local-only (VS Code globalStorage)
• API keys, prompts, responses are never stored
• Storage failures don't affect LLM task execution
• Corrupt data is quarantined, not deleted
• Clear requires nonce-based confirmation

Alternatives Considered

1. Mutable cumulative summary — Simpler but no audit trail, can't reaggregate
2. Scan existing task history on demand — Slow, incomplete data, privacy concerns

Additional Context

• Implementation follows the existing Zoo Code architecture patterns
• No new runtime dependencies (uses existing Zod, proper-lockfile, safeWriteJson)
• Compatible with all 30+ providers through the common ApiStreamUsageChunk type