Should we warn about (potentially) unlisted `hostSpecificFiles`?

Host-specific files that are not listed in `hostSpecificFiles` will be installed on other hosts too, which often (but not always) is a programming error. Unfortunately, we cannot detect host-specific files automatically by their name alone (on any machine other than the host they are intended for), so we'd have to implement this as a form of "lint rule", that emits a warning whenever a dotfile is installed that...

- contains the current hostname
- is not listed under `hostSpecificFiles`

There may, however, be valid use cases for installing files as-is, especially if no further meaning is assigned to file names containing the hostname, in which case such a warning might become noisy. We could add a flag for suppressing it to the `dotpkg.json`, though the question remains as to whether the use case of host-specific files is compelling enough that all file names should be interpreted at all by default, even if only for a warning.

Start by locating the dotfile installation logic and the handling of `hostSpecificFiles`. Review how hostname-containing dotfiles are currently treated, then determine whether a warning and an opt-out belong in the package configuration. Done is not defined until the warning behavior and suppression policy are settled.

Written by the indexing model from the issue text.