Safer handling of unreadable settings files when setting up a home folder

Claude Code can seed a home folder, meaning it writes starting settings into it. That step now handles awkward existing settings files more carefully:

• A settings file that is not an ordinary file, or that cannot be read, is reported as unreadable.
• When some settings were refused, the new settings are rebuilt against the file already there rather than written as first prepared.
• A file that cannot be read in full is not replaced, and Claude Code logs that it could not be replaced.
• When an existing settings file that came from elsewhere is overwritten, the record of the file's contents after the write is left empty.

Before, the step only reported when removing a file failed, and wrote the settings as first prepared.