Ship shell defaults via /etc/skel-devbox so user files are preserved

Previous behaviour (e4063b5) COPY'd .bash_aliases and .inputrc
directly into /home/developer/ during image build. That silently
shadowed any host bind-mount or in-container customization for users
upgrading from v1.14.19b — if you'd written your own .bash_aliases
and rebuilt the container, our baked version would overwrite it
without warning.

Ship the files to /etc/skel-devbox/ instead. The entrypoint copies
them to $HOME only if the target file does not already exist, so:

- Fresh containers get the defaults automatically (unchanged)
- Host bind-mounts win (they materialize before the entrypoint runs)
- Existing in-container customizations survive upgrades
- Defaults remain discoverable at /etc/skel-devbox/ for anyone who
  wants to copy, diff, or reset back to upstream

Docs (README.md, DOCKER_HUB.md, deploy/README.md) describe the new
skel layout and the restore/diff commands.

entrypoint-user.sh

@@ -1,6 +1,20 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
+# ── Shell defaults: copy baked files from /etc/skel-devbox/ if absent
+# Respects host bind-mounts and user customizations — existing files
+# are never overwritten. To restore defaults: rm ~/.bash_aliases (or
+# .inputrc) and recreate the container, or cp from /etc/skel-devbox/
+# directly.
+SKEL_DIR="/etc/skel-devbox"
+if [ -d "$SKEL_DIR" ]; then
+  for f in .bash_aliases .inputrc; do
+    if [ -f "$SKEL_DIR/$f" ] && [ ! -e "$HOME/$f" ]; then
+      cp "$SKEL_DIR/$f" "$HOME/$f"
+    fi
+  done
+fi
+
 # ── Git config defaults ──────────────────────────────────────────────
 if [ -n "${GIT_USER_NAME:-}" ] && ! git config --global user.name &>/dev/null; then
   git config --global user.name "$GIT_USER_NAME"