feat: bake global gitignore (core.excludesFile) into image

Seed ~/.gitignore_global from /etc/skel-devbox (seed-if-absent, like
.bash_aliases/.inputrc, so user edits survive recreate) and wire it via
git config --global core.excludesFile, guarded so a user-set excludesFile
is never overridden. Ignores *.bak, *.bak.*, *~, *.orig, *.swp, *.tmp
across all repos without per-repo .gitignore entries.

entrypoint-user.sh

@@ -33,7 +33,7 @@ fi
 # directly.
 SKEL_DIR="/etc/skel-devbox"
 if [ -d "$SKEL_DIR" ]; then
-  for f in .bash_aliases .inputrc; do
+  for f in .bash_aliases .inputrc .gitignore_global; do
     if [ -f "$SKEL_DIR/$f" ] && [ ! -e "$HOME/$f" ]; then
       cp "$SKEL_DIR/$f" "$HOME/$f"
     fi
@@ -92,6 +92,12 @@ fi
 if [ -n "${GIT_USER_EMAIL:-}" ] && ! git config --global user.email &>/dev/null; then
   git config --global user.email "$GIT_USER_EMAIL"
 fi
+# Global gitignore for personal/tooling artifacts (*.bak, *~, *.orig, ...).
+# Seeded above into $HOME/.gitignore_global from /etc/skel-devbox. Point git at
+# it only if the user has not already set their own core.excludesFile.
+if [ -f "$HOME/.gitignore_global" ] && ! git config --global core.excludesFile &>/dev/null; then
+  git config --global core.excludesFile "$HOME/.gitignore_global"
+fi
 
 # ── Generate opencode config from env vars if no config mounted ──────
 # Delegated to a standalone Python script for clarity and testability.