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>

iso/build-niri-live-iso.sh

@@ -264,6 +264,12 @@ EOF
 install -d -m 0755 "$INCLUDE_DIR/etc/skel/.config/nixpkgs"
 echo '{ allowUnfree = true; }' > "$INCLUDE_DIR/etc/skel/.config/nixpkgs/config.nix"
 
+# git: use file-based credential store so push/pull never tries to open /dev/tty
+cat > "$INCLUDE_DIR/etc/gitconfig" <<'EOF'
+[credential]
+	helper = store
+EOF
+
 # ── 3e) niri config.kdl in /etc/skel ───────────────────────────────────
 # dracut's adduser.sh copies skel → /home/live, so the live user gets a
 # ready niri config without any first-boot setup step.