feat: Lab/CTF creation mode for building custom security challenges

## Summary

Add the ability for users to create and manage custom labs, similar to platforms like HackTheBox, TryHackMe, or CTF competitions, using vt as the underlying infrastructure.

## Use Cases

- **Security trainers** creating custom lab environments for workshops
- **CTF organizers** spinning up challenge infrastructure
- **Companies** building internal security training programs
- **Educators** designing hands-on security courses

## Proposed Features

### 1. Lab Configuration File (`lab.yaml`)

Define a lab with multiple templates, objectives, and hints:

```yaml
name: "SQL Injection Basics"
description: "Learn SQL injection from basics to advanced"
difficulty: easy
duration: 2h

challenges:
- id: challenge-1
template: vt-dvwa
objective: "Find the admin password using SQL injection"
hints:
- "Try single quote in the input field"
- "UNION SELECT can help you extract data"
flag: "FLAG{sql_injection_master}"
points: 100

- id: challenge-2
template: vt-juice-shop
objective: "Bypass the login page"
flag: "FLAG{authentication_bypassed}"
points: 200
```

### 2. CLI Commands

```bash
# Create a new lab from config
vt lab create --config lab.yaml

# Start a lab session
vt lab start --name "SQL Injection Basics"

# List available labs
vt lab list

# Check progress / submit flags
vt lab submit --challenge challenge-1 --flag "FLAG{...}"

# Stop all lab containers
vt lab stop --name "SQL Injection Basics"

# Export lab for sharing
vt lab export --name "SQL Injection Basics" --output lab.zip
```

### 3. Lab Filtering & Discovery

```bash
# Filter labs by difficulty
vt lab list --difficulty easy

# Filter by tags
vt lab list --tags web,sqli

# Filter by estimated duration
vt lab list --duration 1h
```

### 4. Progress Tracking

- Track completed challenges per lab
- Score/points system
- Time tracking for speedrun mode
- Leaderboard export (for CTF competitions)

## Benefits

- **Reusable**: Labs can be shared and reused across teams
- **Portable**: Export/import labs for offline use
- **Scalable**: Run multiple instances for competitions
- **Integrated**: Leverages existing vt-templates ecosystem

## Open Questions

- Should labs support multi-user sessions with isolated environments?
- Integration with flag submission APIs for CTF platforms?
- Support for dynamic flag generation?

## Related

- vt-templates repository for base vulnerable environments
- Could integrate with existing CTF frameworks (CTFd, etc.)