fix: skip missing authorized_keys cleanup paths

[nanookclaw]

Closes #709.

This updates the per-user authorized_keys cleanup so the find command starts at the user home directory and filters to .ssh/authorized_keys, instead of using the file itself as the find starting path. When a user does not have that file, find now exits successfully with no matches instead of failing the whole cleanup script under set -e.

The follow-up verification loop is unchanged, so the script still fails if an authorized_keys file remains after cleanup.

Testing:

• bash -n scripts/cleanup.sh
• git diff --check
• temporary find smoke check with set -e for both missing and present .ssh/authorized_keys

[nanookclaw]

Rebased fix/cleanup-missing-authorized-keys onto current main and force-pushed head e0077b9.

Verification after the rebase:

• bash -n scripts/cleanup.sh
• git diff --check origin/main...HEAD
• smoke check for the updated find /home/<user> -maxdepth 2 -path .../.ssh/authorized_keys behavior with both missing and present authorized_keys