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.

Dockerfile.base

@@ -427,8 +427,9 @@ ENV PATH="/home/${USER_NAME}/.config/opencode/npm-global/bin:${PATH}"
 
 # ── Shell defaults (bash history, aliases, readline) ─────────────────
 RUN mkdir -p /etc/skel-devbox
-COPY rootfs/home/developer/.bash_aliases /etc/skel-devbox/.bash_aliases
-COPY rootfs/home/developer/.inputrc      /etc/skel-devbox/.inputrc
+COPY rootfs/home/developer/.bash_aliases     /etc/skel-devbox/.bash_aliases
+COPY rootfs/home/developer/.inputrc          /etc/skel-devbox/.inputrc
+COPY rootfs/home/developer/.gitignore_global /etc/skel-devbox/.gitignore_global
 
 # ── Entrypoint ────────────────────────────────────────────────────────
 COPY rootfs/usr/local/lib/opencode-devbox/ /usr/local/lib/opencode-devbox/