chore: update GNS3 skills repository to official organization

Update the default download repository address for GNS3 skills from
yueguobin/GNS3-Skills to gns3/gns3-skills to use the official
organization repository.

This affects:
- Default skills_repo_url in server configuration schema
- Skills manager default repository URL
- Skills configuration defaults
- All documentation references

gns3server/agent/gns3_copilot/configs/skills_config.py

@@ -34,7 +34,7 @@ from gns3server.config import Config
 # Default skills repository configuration
 SKILLS_CONFIG = {
     # Git repository URL for skills
-    "repo_url": "https://github.com/yueguobin/GNS3-Skills.git",
+    "repo_url": "https://github.com/gns3/gns3-skills.git",
 
     # Git branch to use
     "branch": "main",

gns3server/agent/gns3_copilot/prompts/__init__.py

@@ -29,7 +29,7 @@ Prompts Module for GNS3-Copilot
 This package provides system prompts loading utilities for the GNS3-Copilot AI agent.
 
 All system prompts are now loaded from the external GNS3-Skills repository:
-https://github.com/yueguobin/GNS3-Skills
+https://github.com/gns3/gns3-skills
 
 Available prompts (loaded from external repository):
 - lab_automation_assistant.md: Lab automation mode (diagnostics + config)

gns3server/agent/gns3_copilot/skills/manager.py

@@ -76,12 +76,12 @@ class SkillsManager:
         Initialize the skills manager.
 
         Args:
-            repo_url: Git repository URL (default: https://github.com/yueguobin/GNS3-Skills.git)
+            repo_url: Git repository URL (default: https://github.com/gns3/gns3-skills.git)
             branch: Git branch to use (default: "main")
             auto_update: Whether to automatically pull updates on reload
         """
         if repo_url is None:
-            repo_url = "https://github.com/yueguobin/GNS3-Skills.git"
+            repo_url = "https://github.com/gns3/gns3-skills.git"
 
         # Get local path from GNS3 config directory
         config_dir = Config.instance().config_dir