Swap real keystroke for a virtual one

Having rebound `M+h` to moving focus west, I realised how much I miss the hide functionality! You can rebind it using System Preferences, but only on an app-by-app basis, eg Safari > Hide Safari. This is impractical so I was wondering if it's possible to bind another keystroke, say `S-M-m`, and have `skhd` capture it and send a 'virtual' `M+h` to the OS? So Mac OS would still think that `M+h` was pressed and hide the appropriate app, but the physical combination of Cmd and H could be kept for navigation.

The issue names no files or tests. Start by tracing how skhd captures and dispatches key bindings, then investigate the macOS event APIs needed to synthesize Cmd-H; done means a new binding hides the active application while physical Cmd-H remains available for navigation.

Written by the indexing model from the issue text.