Simplify authentication to require URL and API key only

Major configuration and tooling updates:

Authentication Changes:
- Remove username/password authentication support
- Require PORTAINER_URL and PORTAINER_API_KEY (both mandatory)
- Simplify PortainerConfig class and validation logic
- Update all documentation to reflect API key requirement

Multiple Runtime Support:
- Add uvx support for running without installation
- Add uv support with dedicated wrapper script
- Add npx support with Node.js wrapper script
- Maintain backward compatibility with direct Python execution

Documentation Updates:
- Comprehensive README.md with all execution methods
- Detailed USAGE.md with step-by-step instructions
- Updated .env.example with clear required vs optional sections
- Enhanced docstrings in server.py and config.py

Tooling Support:
- package.json for npm/npx support with cross-platform wrapper
- scripts/run-with-uv.py for uv integration
- bin/portainer-core-mcp Node.js wrapper for npx
- test_uvx.py for uvx functionality testing

Configuration Improvements:
- Clear separation of required vs optional environment variables
- Better validation error messages
- Simplified authentication flow
- Enhanced project metadata in pyproject.toml

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

.env.example

@@ -1,15 +1,24 @@
 # Portainer Core MCP Server Configuration
 
-# Portainer connection settings
-PORTAINER_URL=https://your-portainer-instance.com
+# =============================================================================
+# REQUIRED CONFIGURATION
+# =============================================================================
 
-# Authentication settings (choose one method)
-# Method 1: API Key authentication (recommended)
+# Portainer instance URL (required)
+# Examples:
+#   - https://portainer.example.com
+#   - https://portainer.company.com:9443
+#   - http://localhost:9000
+PORTAINER_URL=https://portainer.example.com
+
+# Portainer API key for authentication (required)
+# Generate this from Portainer UI: User settings > API tokens
+# Example: ptr_XYZ123abc456def789
 PORTAINER_API_KEY=your-api-key-here
 
-# Method 2: Username/Password authentication
-# PORTAINER_USERNAME=admin
-# PORTAINER_PASSWORD=your-password-here
+# =============================================================================
+# OPTIONAL CONFIGURATION
+# =============================================================================
 
 # HTTP client settings
 HTTP_TIMEOUT=30
@@ -31,6 +40,3 @@ LOG_FORMAT=json
 # Development settings
 DEBUG=false
 
-# Server settings
-SERVER_HOST=localhost
-SERVER_PORT=8000