[codex] Add Rectangle with window manager conflict handling

[felipe0328]

What changed

• add Rectangle as an optional Homebrew cask component
• introduce reusable component conflict metadata and validation
• warn interactive users who select both Rectangle and AeroSpace, then ask which one to keep
• reject conflicting non-interactive selections
• prefer the recommended window manager in --all mode
• replace a conflicting saved window manager when users switch choices
• document the Rectangle/AeroSpace relationship

Why

Rectangle and AeroSpace both manage windows and overlapping controls can produce a confusing setup. Users should be able to install Rectangle while receiving clear guidance that the two alternatives should not run together.

Impact

AeroSpace remains selected by default. Rectangle is available in the installer but starts unselected. If both are selected interactively, installation pauses and asks the user which one to keep before making changes.

Validation

• bash tests/installer_test.sh — 27 tests passed
• shellcheck -x lib/core.sh lib/ui.sh install.sh sync.sh components/42-rectangle.sh tests/installer_test.sh
• bash -n lib/core.sh lib/ui.sh install.sh sync.sh components/42-rectangle.sh tests/installer_test.sh
• git diff --check

[github-actions]

Version preview

Current	Expected after merge	Bump
1.2.1	1.3.0	minor

This PR will trigger a version bump after it is merged into main.

conventional commits require minor bump.

[chatgpt-codex-connector]

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