Add automatic CUA_API_KEY environment variable support for Computer and CloudProvider

Previously, users had to explicitly read the CUA_API_KEY environment variable
and pass it to the Computer/CloudProvider constructor. This change makes the
API key parameter optional and automatically reads from the CUA_API_KEY
environment variable if not provided.

Changes:
- CloudProvider.__init__: Made api_key parameter optional, reads from CUA_API_KEY env var
- Computer.__init__: Added fallback to CUA_API_KEY env var for api_key parameter
- Updated documentation examples to show simplified usage without explicit api_key
- Updated cloud_api_examples.py to demonstrate the new simpler pattern

This provides a cleaner API while maintaining backward compatibility with
explicit api_key parameter passing.

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

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

examples/cloud_api_examples.py

@@ -9,14 +9,13 @@ from computer.providers.cloud.provider import CloudProvider
 
 
 async def main() -> None:
-    api_key = os.getenv("CUA_API_KEY")
-    if not api_key:
-        raise RuntimeError("CUA_API_KEY environment variable is not set")
+    # CloudProvider will automatically read CUA_API_KEY from environment if not provided
+    # You can still pass api_key explicitly if needed: CloudProvider(api_key="your_key")
     api_base = os.getenv("CUA_API_BASE")
     if api_base:
         print(f"Using API base: {api_base}")
 
-    provider = CloudProvider(api_key=api_key, verbose=True)
+    provider = CloudProvider(verbose=True)
     async with provider:
 
         # List all VMs