fix: untrack runtime config files and add them to .gitignore

These files are machine-specific and generated at runtime — they should
never have been committed. Remove from index (files kept on disk) and
add explicit gitignore rules to prevent future re-adds.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/mail/mailserver.env

@@ -1,3 +0,0 @@
-OVERRIDE_HOSTNAME=mail.cell.local
-POSTMASTER_ADDRESS=admin@cell.local
-LOG_LEVEL=warn