Redesign wizard UI with gum (charmbracelet)

- Bootstrap gum automatically on first run (Arch/Debian/RHEL/Fedora/SUSE)
- utils.sh: replace all bash color helpers with gum equivalents
  - gum input for text prompts (with value pre-fill for defaults)
  - gum choose for selection menus
  - gum confirm for yes/no
  - gum spin for long-running operations
  - gum style/log for output (catppuccin mocha palette)
  - gum style for banners and summary box
- core.sh: spinner on git clone/pull
- workflow.sh: spinner on git clone
- prereqs.sh: spinner on package installs
- wizard.sh: double-border welcome banner, rounded summary box,
  success banner with next-steps panel

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

lib/core.sh

@@ -9,12 +9,11 @@ setup_core() {
     header "Context Studio Core"
 
     if [[ -d "$CS_CORE_DIR/.git" ]]; then
-        success "Core already installed at $CS_CORE_DIR"
-        ask_yn _update "Update core to latest?" "n"
+        success "Core installed at $CS_CORE_DIR"
+        ask_yn _update "Pull latest updates?" "n"
         if [[ "$_update" == "y" ]]; then
-            info "Pulling latest core..."
-            git -C "$CS_CORE_DIR" pull --ff-only \
-                || warn "Pull failed — continuing with existing version"
+            spin "Updating core..." git -C "$CS_CORE_DIR" pull --ff-only \
+                || warn "Pull failed — continuing with current version."
             success "Core updated"
         fi
         return 0
@@ -22,7 +21,8 @@ setup_core() {
 
     info "Cloning context-studio-core → $CS_CORE_DIR"
     mkdir -p "$CS_HOME"
-    git clone "$CS_CORE_REPO" "$CS_CORE_DIR" \
+    spin "Cloning context-studio-core..." \
+        git clone "$CS_CORE_REPO" "$CS_CORE_DIR" \
         || die "Failed to clone context-studio-core. Check your SSH key and network."
     success "Core installed at $CS_CORE_DIR"
 }