feat: add fault injection system and external skills repository

## Summary

Add a complete fault injection system for GNS3 Copilot, migrate all
skills from local Python files to an external Git repository with
hot reload support, and restructure Copilot API under /copilot/.

## Key Changes

### Fault Injection
- New troubleshooting_injection mode with InjectionSkillsTool
- 368 fault scenarios across 39 protocol categories
- Context-based filtering (LLM must pass topology protocols)

### External Skills Repository
- SkillsManager: Git clone/pull, version tracking, smart updates
- SkillsLoader: YAML skills + Markdown prompts from external repo
- Hot reload via POST /copilot/reload/skills
- Configurable via gns3_server.conf

### Architecture
- API unified under /copilot/ prefix
- SkillsManager moved from Controller to agent module
- Lazy initialization with startup background preload
- Per-command Git timeout, smart update checks
- Forbidden commands hot-reloadable from external repo
- 32 INFO logs downgraded to DEBUG

gns3server/config_samples/gns3_server.conf

@@ -113,6 +113,14 @@ enable_builtin_templates = True
 ; Install built-in appliances
 install_builtin_appliances = True
 
+; Git repository URL for GNS3 Copilot external skills
+; This repository provides injection skills, prompts, and device skills
+; skills_repo_url = https://github.com/yueguobin/GNS3-Skills.git
+; Git branch for the skills repository
+; skills_repo_branch = main
+; Automatically pull updates from the skills repository when reloading
+; skills_auto_update = false
+
 ; check if hardware virtualization is used by other emulators (KVM, VMware or VirtualBox)
 hardware_virtualization_check = True