Configure git credential.helper=store system-wide

Prevents git from trying to open /dev/tty for password prompts in
environments without a controlling terminal (live ISO, scripts, Claude Code).
Credentials stored once in ~/.git-credentials and reused automatically.

Applied to: both live ISO include overlays and the installer target system.

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

installer/lib/postinstall.sh

@@ -88,6 +88,12 @@ EOF
     fi
     chmod 440 "$TARGET/etc/sudoers.d/10-wheel"
 
+    # git: use file-based credential store so push/pull never tries to open /dev/tty
+    cat > "$TARGET/etc/gitconfig" <<'EOF'
+[credential]
+	helper = store
+EOF
+
     ok "user '$USERNAME' created and added to: $USER_GROUPS"
 }