feat: Add RegistryAuth::from_default() convenience methods

- Add from_default() and from_default_str() methods to RegistryAuth
- These methods automatically resolve auth from Docker config and credential helpers
- Update example to demonstrate both explicit and auto auth approaches
- Simplify krust's auth wrapper to use the new convenience method
- Fix test environment variable pollution

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

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

src/auth/simple.rs

@@ -1,11 +1,10 @@
 //! Simple authentication wrapper using oci-distribution's credential helper
 
 use anyhow::Result;
-use oci_distribution::credential_helper::resolve_docker_auth;
 use oci_distribution::secrets::RegistryAuth;
 
 /// Resolve authentication for a given resource using Docker config and credential helpers
 pub fn resolve_auth(resource: &str) -> Result<RegistryAuth> {
-    resolve_docker_auth(resource)
+    RegistryAuth::from_default_str(resource)
         .map_err(|e| anyhow::anyhow!("Failed to resolve auth: {}", e))
 }