feat: change /opt to symlink

[NN708]

Resolves #497.

[taukakao]

I strongly disagree with this. It doesn't fix anything and just makes custom images less approachable for less experienced users and more annoying for all users.

[NN708]

@taukakao Thanks for your review, but I think using OverlayFS for /opt is a worse approach. We need clear boundaries between directories managed by the system image developer and those managed by the end user. Without this, users may modify files and then never receive updates, leading to confusion and unexpected behavior. Apart from /etc (where we must provide defaults), /opt is currently the only directory still relying on OverlayFS.

A major reason Linux can be difficult to use is its excessive freedom: a lack of enforced standards, and even where standards exist, many people ignore them. With immutability, we have already established a clear separation between the system and applications. System image developers should ensure that system packages are always installed to /usr, not /opt. Other immutable distros already symlink /opt to /var/opt, and I don't see a compelling reason for Vanilla OS to deviate from that practice.

[taukakao]

@NN708 It's much harder to modify the system image for most other immutable distros, which is a big selling point for VOS in my eyes.

If this is only about boundaries, then we could implement a method where the image can define what folders in /opt are mounted from the image, and the rest would be up to the user.

[NN708]

It's much harder to modify the system image for most other immutable distros, which is a big selling point for VOS in my eyes.

That's true, but system image developers are still technical users who will read our docs. Avoiding confusion for end users is more important than making things slightly easier for developers.

If this is only about boundaries, then we could implement a method where the image can define what folders in /opt are mounted from the image, and the rest would be up to the user.

This introduces unnecessary complexity. It's simpler for system image developers to read "/opt, /srv and /usr/local are reserved for local administrators" than to figure out how to define mount points. This approach would also confuse end users when they switch between different images or come from another immutable distro.