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>

commands/usage.md

@@ -0,0 +1,66 @@
+---
+name: usage
+description: View model usage statistics and session history
+aliases: [stats]
+invokes: skill:usage
+---
+
+# /usage Command
+
+View usage statistics across Claude Code sessions.
+
+## Usage
+
+```
+/usage                    # Summary (last 7 days)
+/usage today              # Today's sessions
+/usage week               # Last 7 days (default)
+/usage month              # Last 30 days
+/usage all                # All time
+```
+
+## Grouping
+
+```
+/usage --by agent         # Group by agent
+/usage --by skill         # Group by skill
+/usage --by model         # Group by model tier
+```
+
+## Configuration
+
+```
+/usage --set-level minimal|standard|detailed
+/usage --show-config
+```
+
+## Log Levels
+
+| Level | Data Captured |
+|-------|---------------|
+| `minimal` | Session ID, time, model |
+| `standard` | + agents, skills, tokens (default) |
+| `detailed` | + commands, delegations, errors |
+
+## Example Output
+
+```
+📊 Usage Summary — Last 7 Days
+
+Sessions: 12
+Total time: 8h 32m
+Model: opus (primary)
+
+┌─────────────┬──────────┬────────┐
+│ Agent       │ Sessions │ Time   │
+├─────────────┼──────────┼────────┤
+│ PA          │ 12       │ 8h 32m │
+│ sysadmin    │ 4        │ 1h 15m │
+└─────────────┴──────────┴────────┘
+```
+
+## Data Sources
+
+- Session metadata from `~/.claude/state/personal-assistant/history/index.json`
+- Configuration from `~/.claude/state/usage/config.json`
+- Session content from `history/*.jsonl` (when available)