Add sidebar tab context menu actions

[nicosuave]

• add Close Tab to sidebar tab rows and route it to the exact native tab window behind the row
• add the same Remove Worktree… and Reveal in Finder actions used by regular worktree rows
• keep the existing remove-worktree disable behavior for main worktrees

Verified with zig build test.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Close tab when no native tab group exists

The new sidebar Close Tab action becomes a no-op when the current window is not in an AppKit tab group (for example, a single window/tab setup), because closeNativeTab returns early on window.tabGroup == nil instead of delegating to closeTab(_:). This means users can see the context-menu action but it silently does nothing in that common state.

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