`chezmoi apply` on Linux printed "could not regenerate kitty theme
files" every run. GNU stat reads -f as --file-system, so
`stat -f '%Lp' file` prints a block of filesystem info to stdout and
exits non-zero. The `|| stat -c '%a'` fallback then appended the real
answer to that block, and chmod rejected the result, failing the script
under set -e.
Replace the chain with a file_mode() helper in both scripts: try the GNU
form first, since BSD rejects -c cleanly without printing anything, and
accept the answer only if it is octal. Covered by a new case in
tests/test_theme_mode.sh using a stat stub that reproduces GNU's
behaviour; it fails against the old chain.
theme-mode is a per-machine preference, and the three *-theme.auto.conf
files are generated from it plus themes/, so tracking any of them made
every machine flag the others' choice. Drop all four from the source
state and ignore them.
That leaves a freshly applied machine with no theme files for kitty to
read, so add run_after_kitty-theme-mode.sh to regenerate them from the
canonical themes. It defaults to `auto`, so a new machine follows its OS
until told otherwise, and it exits quietly where this kitty config isn't
applied rather than failing the run.
Since that script now runs on every apply, including on Linux, make the
permission lookup in both shell scripts fall back from BSD `stat -f` to
GNU `stat -c` and then to 600.
