Add bootstrap.sh with zsh and git modules

Cross-distro (RHEL/Debian) VM setup: zsh + oh-my-zsh with a self-healing
bash fallback for environments that reset the root filesystem or hardcode
bash for new terminals (e.g. Kasm Workspaces), plus git identity and
HTTPS credential-store configuration.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/os-detect.sh

@@ -0,0 +1,43 @@
+#!/usr/bin/env sh
+# lib/os-detect.sh — shared OS/package-manager detection for module installers.
+# Source this, then call pkg_install <pkg> [pkg...].
+
+detect_pkg_family() {
+  if [ -f /etc/os-release ]; then
+    # shellcheck disable=SC1091
+    id_line="$(. /etc/os-release && printf '%s %s' "${ID:-}" "${ID_LIKE:-}")"
+    case "$id_line" in
+      *debian*|*ubuntu*) printf 'debian\n'; return 0 ;;
+      *rhel*|*fedora*|*centos*) printf 'rhel\n'; return 0 ;;
+    esac
+  fi
+  if command -v apt-get >/dev/null 2>&1; then
+    printf 'debian\n'; return 0
+  fi
+  if command -v dnf >/dev/null 2>&1 || command -v yum >/dev/null 2>&1; then
+    printf 'rhel\n'; return 0
+  fi
+  printf 'unknown\n'
+  return 1
+}
+
+pkg_install() {
+  family="$(detect_pkg_family)"
+  case "$family" in
+    debian)
+      sudo apt-get update -qq
+      sudo apt-get install -y "$@"
+      ;;
+    rhel)
+      if command -v dnf >/dev/null 2>&1; then
+        sudo dnf install -y "$@"
+      else
+        sudo yum install -y "$@"
+      fi
+      ;;
+    *)
+      printf 'error: unsupported/undetected OS family; install manually: %s\n' "$*" >&2
+      return 1
+      ;;
+  esac
+}