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>

skills/usage/SKILL.md

@@ -0,0 +1,153 @@
+---
+name: usage
+description: Track and report model usage across sessions
+allowed-tools:
+  - Bash
+  - Read
+---
+
+# Usage Tracking Skill
+
+Query session history and report usage statistics.
+
+## Data Sources
+
+| Source | Status | Data Available |
+|--------|--------|----------------|
+| `history/index.json` | Available | Session IDs, start times |
+| `history/*.jsonl` | Future | Session content, commands, tokens |
+| `usage/config.json` | Available | Log level, preferences |
+
+## Command Routing
+
+| Command | Action |
+|---------|--------|
+| `/usage` | Show last 7 days summary |
+| `/usage today` | Today's sessions |
+| `/usage week` | Last 7 days |
+| `/usage month` | Last 30 days |
+| `/usage all` | All time stats |
+| `/usage --by agent` | Group by agent |
+| `/usage --by skill` | Group by skill |
+| `/usage --by model` | Group by model tier |
+| `/usage --set-level <level>` | Set log level |
+| `/usage --show-config` | Show current config |
+
+## Log Levels
+
+| Level | Data Captured |
+|-------|---------------|
+| `minimal` | Session ID, time, model |
+| `standard` | + agents, skills, tokens (default) |
+| `detailed` | + commands, delegations, errors |
+
+## Implementation
+
+### Query Sessions
+
+```python
+import json
+from datetime import datetime, timedelta
+from pathlib import Path
+from zoneinfo import ZoneInfo
+
+LOCAL_TZ = ZoneInfo('America/Los_Angeles')
+HISTORY_DIR = Path.home() / ".claude/state/personal-assistant/history"
+CONFIG_PATH = Path.home() / ".claude/state/usage/config.json"
+
+def load_sessions():
+    index_path = HISTORY_DIR / "index.json"
+    if not index_path.exists():
+        return []
+    with open(index_path) as f:
+        data = json.load(f)
+    return data.get("sessions", [])
+
+def filter_sessions(sessions, range_type="week"):
+    now = datetime.now(LOCAL_TZ)
+    if range_type == "today":
+        cutoff = now.replace(hour=0, minute=0, second=0, microsecond=0)
+    elif range_type == "week":
+        cutoff = now - timedelta(days=7)
+    elif range_type == "month":
+        cutoff = now - timedelta(days=30)
+    else:  # all
+        return sessions
+
+    filtered = []
+    for s in sessions:
+        started = datetime.fromisoformat(s["started"])
+        if started >= cutoff:
+            filtered.append(s)
+    return filtered
+
+def estimate_duration(sessions):
+    """Estimate session duration from consecutive start times."""
+    if not sessions:
+        return []
+
+    sorted_sessions = sorted(sessions, key=lambda s: s["started"])
+    for i, session in enumerate(sorted_sessions):
+        if i + 1 < len(sorted_sessions):
+            start = datetime.fromisoformat(session["started"])
+            next_start = datetime.fromisoformat(sorted_sessions[i + 1]["started"])
+            duration = (next_start - start).total_seconds() / 60
+            # Cap at 2 hours (likely session gap)
+            session["duration_mins"] = min(duration, 120)
+        else:
+            # Current/last session: estimate 30 mins
+            session["duration_mins"] = 30
+    return sorted_sessions
+```
+
+### Generate Report
+
+```python
+def generate_report(sessions, range_type="week"):
+    total_mins = sum(s.get("duration_mins", 0) for s in sessions)
+    hours = int(total_mins // 60)
+    mins = int(total_mins % 60)
+
+    report = f"""
+📊 Usage Summary — {range_type.title()}
+
+Sessions: {len(sessions)}
+Total time: {hours}h {mins}m
+Model: opus (primary)
+"""
+    return report
+```
+
+## Output Format
+
+```
+📊 Usage Summary — Last 7 Days
+
+Sessions: 12
+Total time: 8h 32m
+Model: opus (primary)
+
+┌─────────────┬──────────┬────────┐
+│ Agent       │ Sessions │ Time   │
+├─────────────┼──────────┼────────┤
+│ PA          │ 12       │ 8h 32m │
+└─────────────┴──────────┴────────┘
+
+Note: Detailed metrics available when session
+content logging is enabled.
+```
+
+## Future Enhancements
+
+When `history/*.jsonl` files are available:
+- Parse actual commands used
+- Detect skill invocations
+- Count delegations (sonnet/haiku)
+- Estimate token usage
+- Track errors
+
+## Policy
+
+- Read-only operations
+- No external API calls
+- Local data only