Add standard shelf keyboard actions

[brzvsk]

Hi! A little backstory: my Paste subscription came up for renewal, and I realized I was not ready to keep paying for it. I asked my Codex agent to look for alternatives; among several similar projects, Copy was the one I liked most. I would be happy to contribute improvements here—straightforward fixes as pull requests, and more opinionated ideas as issues.

What changed

• Backspace deletes the selected shelf card while preserving normal search editing
• Cmd+C copies the primary card to the clipboard and closes the shelf on success
• Cmd+V pastes the selected card
• Cmd+, opens the real Settings window instead of the placeholder SwiftUI scene
• README documents the new shelf shortcuts
• A successful shelf Cmd+C emits a lightweight completion event used by optional feedback in Add optional clipboard capture sounds #3; without Add optional clipboard capture sounds #3 it is a no-op

Testing

• swift test --package-path CopyCore --disable-automatic-resolution — 184 tests passed
• Debug app target builds successfully with Xcode 26.6
• Installed and launched a locally signed Debug build using the existing bundle identifier