Switch to controller-runtime SSA for resource management

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This PR migrates the GitOpsSet controller from traditional Create/Patch operations to Server-Side Apply (SSA) using controller-runtime's built-in SSA support.

Changes Made

The core change is in the renderAndReconcile function where the complex resource management logic has been simplified:

Before: Complex logic with separate Create/Update paths

if existingEntries.Has(ref) {
    existing, err := unstructuredFromResourceRef(ref)
    // ... complex merge logic with Get, Patch
    newResource = copyUnstructuredContent(existing, newResource)
    k8sClient.Patch(ctx, newResource, client.MergeFrom(existing))
} else {
    k8sClient.Create(ctx, newResource)
}

After: Simple, unified server-side apply

// Use server-side apply for both create and update operations
k8sClient.Patch(ctx, newResource, client.Apply, client.FieldOwner("gitopssets-controller"))

Benefits

• Simplified Code: Reduced resource management logic from 50+ lines to 4 lines
• Better Conflict Resolution: Server handles conflicts automatically with proper field ownership
• Field Ownership Tracking: Resources are properly tagged with "gitopssets-controller" field manager
• Atomic Operations: Eliminates race conditions between Get/Create/Update operations
• More Reliable: Server-side apply handles edge cases better than client-side logic

Additional Changes

• Removed the copyUnstructuredContent helper function (no longer needed)
• Removed unused apierrors import
• Updated log messages from "creating new resource" to "applying resource"

The implementation follows the official controller-runtime SSA documentation referenced in the issue and represents a significant improvement in reliability and maintainability.

Fixes #115.

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