implement category priorities

[marinjurjevic]

Motivation

This PR introduces category priorities. This can be useful when you need to override a common tool setting with machine specific setting. In general, it allows a way to make category take precedence by listing it after another category.

Most people use dotgit restore without explicit category, where common and hostname categories are implicitly added. common category has lowest priority because it's added first. hostname category has highest priority because it is appended last.

For example:

computer=alias,zsh,vim,git

.profile
.profile:computer

.zshenv:zsh
.zshenv:computer

Both of these file configs cause error.

First one is problematic when restoring without explicit category.

$> dotgit restore 
multiple category lists active for .profile: ['common'] and ['computer'] 

Second one fails in similar manner.

$> dotgit restore 
multiple category lists active for .profile: ['zsh'] and ['computer'] 

Implementation

Categories are prioritized based on their order after category list is flattened during activation. This allows fine tuning configurations and having general configs overriden by specific configs.

DISCLAIMER: Code and tests have been generated by AI agent. I've added a sentence do docs more as a placeholder. Maybe cookbook could be expanded with more examples and with more complex filelist examples.