Clipboard restore path clears non-text clipboard content

[xvvcs]

Problem

When command-pipeline capture fails, restoreClipboard(_:) writes an empty string if the previous clipboard content was not a string (nil). This can erase existing non-text clipboard payloads (images/files/rich content).

Impact

• Unexpected clipboard data loss for users.
• Usability regression for workflows relying on non-text clipboard items.

Evidence

• Previous clipboard read as string only: PromptRefactorApp/PromptRefactorApp/AppRuntimeController.swift:306
• Restore behavior writing empty string on nil: PromptRefactorApp/PromptRefactorApp/AppRuntimeController.swift:423

Recommendation

• Preserve full pasteboard state (types/items), not just plain string.
• If full restoration is not feasible, avoid clearing clipboard when original content is non-string.
• Add regression tests for non-text clipboard before/after failed capture.

[xvvcs]

@copilot evaliuate on this please