Fix dev-add.sh to check file readability, not just existence

Change [ -f /etc/slack-dev.env ] to [ -r ... ] so users not in
devs group don't get permission denied errors on login.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

scripts/dev-add.sh

@@ -87,8 +87,8 @@ create_user() {
         echo '# npm global packages (fallback)'
         echo "export PATH=\"\$HOME/.npm-global/bin:\$PATH\""
         echo ''
-        echo '# Slack bot development tokens'
-        echo '[ -f /etc/slack-dev.env ] && source /etc/slack-dev.env'
+        echo '# Slack bot development tokens (if readable)'
+        echo '[ -r /etc/slack-dev.env ] && source /etc/slack-dev.env'
     } > "$profile"
     chown "$username:users" "$profile"