Gate ghostty performKeyEquivalent on actual first-responder (#264)
The cached `focused` flag is updated via `becomeFirstResponder` /
`resignFirstResponder`, but AppKit walks `performKeyEquivalent` over
every view in the window. Clicking a sidebar `List` row takes
selection without always demoting the surface out of the responder
chain, so ⌘⌫ reached `keyDown` and was consumed by the terminal
instead of firing Archive Worktree in the main menu. Check
`window?.firstResponder === self` alongside `focused` so the surface
only processes events AppKit actually routes to it.
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