Initial setup: mac_os installer with custom config

Contains both components needed for Mac setup:
- mac_os/: Installer scripts (from bkuhlmann/mac_os)
- mac_os-config/: Custom app configuration

Configured apps:
- Homebrew formulas: atuin, bash, ffmpeg, mas, mole, node, rename, ykman
- Homebrew casks: chromium, deepl, element, nextcloud, nova, proton suite, signal, transmit
- App Store: Affinity suite, Bitwarden, Final Cut Pro, Invoice Ninja, iWork, PastePal, xSearch

Usage: cd mac_os && ./bin/run

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

mac_os-config/.github/PULL_REQUEST_TEMPLATE.md

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+## Overview
+<!-- Required. Describe, briefly, why this is necessary and what the overarching architecture is. -->
+
+## Screenshots/Screencasts
+<!-- Optional. Provide supporting screen shots/casts. -->
+
+## Details
+<!-- Optional. As bullet points, list related issue(s); major highlights; team callouts; and/or other information that is helpful. -->