Fix set -e bug causing silent exit after valid input

- Replace all `[[ condition ]] && die` with `if/fi` — the && pattern
  exits silently when the condition is false under set -e
- Removed -e from set flags (kept -uo pipefail), all error paths are
  now explicit
- Declare `input` as local in ask/ask_yn/ask_choice to prevent leakage
- Use `read -r input || true` to handle EOF safely
- Fix ask_choice arithmetic to avoid (()) triggering exit on false

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/prereqs.sh

@@ -41,7 +41,7 @@ ensure_git() {
     fi
     warn "git not found."
     ask_yn _install "Install git now?" "y"
-    [[ "$_install" != "y" ]] && die "git is required."
+    if [[ "$_install" != "y" ]]; then die "git is required."; fi
     install_pkg git
     command -v git &>/dev/null || die "git installation failed."
     success "git installed: $(git --version)"
@@ -69,7 +69,8 @@ ensure_container_runtime() {
         "podman (recommended)" \
         "docker"
 
-    case "$_runtime" in
+    local runtime_choice="$_runtime"
+    case "$runtime_choice" in
         podman*)
             _install_podman
             CONTAINER_CMD="podman"