Implement /usage command for session tracking (fc-005)

Components:
- commands/usage.md: Slash command with aliases (stats)
- skills/usage/SKILL.md: Query logic and report generation
- state/usage/config.json: Log level and preferences

Features:
- Parse history/index.json for session metadata
- Estimate duration from consecutive session starts
- Group by date, show summary stats
- Configurable log levels (minimal/standard/detailed)
- Extensible for future session content parsing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

state/usage/config.json

@@ -0,0 +1,8 @@
+{
+  "version": "1.0",
+  "log_level": "standard",
+  "default_range": "week",
+  "track_tokens": true,
+  "model": "opus",
+  "notes": "Session content parsing available when history/*.jsonl files exist"
+}