feat: add Junie agent integration

Add support for JetBrains Junie AI agent:
- Create .junie/guidelines.md with workflow instructions
- Create .junie/mcp/mcp.json for MCP server configuration
- Add 'junie' to BuiltinRecipes in recipes.go
- Add runJunieRecipe() handler in setup.go
- Add website documentation
- Add integrations/junie/README.md

Usage: bd setup junie

website/sidebars.ts

@@ -91,6 +91,7 @@ const sidebars: SidebarsConfig = {
         'integrations/claude-code',
         'integrations/mcp-server',
         'integrations/aider',
+        'integrations/junie',
       ],
     },
     {