feat: implement modular numbered script system

- Add scripts/ directory with numbered execution pattern
- Create 00-check-dependencies.sh for requirement validation
- Create 10-backup-files.sh for conflict handling
- Create 20-setup-stow.sh for symlink creation
- Create 90-post-install.sh for cleanup tasks
- Add scripts/utils.sh with colored logging functions
- Update install.sh to auto-discover and execute numbered scripts
- Remove hardcoded logic in favor of flexible script discovery

Benefits:
- Clean separation of concerns (each script has one job)
- Numbered execution order (SSH/systemd convention)
- Colored output for better UX
- Easy to extend with additional scripts
- Same entry point for all branches

scripts/utils.sh

@@ -0,0 +1,36 @@
+#!/bin/bash
+# scripts/utils.sh - Shared utilities and functions
+
+# Colors for output
+RED='\033[0;31m'
+GREEN='\033[0;32m'
+YELLOW='\033[1;33m'
+BLUE='\033[0;34m'
+NC='\033[0m' # No Color
+
+# Logging functions
+log_info() {
+    echo -e "${BLUE}ℹ️  $1${NC}"
+}
+
+log_success() {
+    echo -e "${GREEN}✅ $1${NC}"
+}
+
+log_warning() {
+    echo -e "${YELLOW}⚠️  $1${NC}"
+}
+
+log_error() {
+    echo -e "${RED}❌ $1${NC}"
+}
+
+# Check if command exists
+command_exists() {
+    command -v "$1" &> /dev/null
+}
+
+# Check if we're in an interactive shell
+is_interactive() {
+    [[ $- == *i* ]]
+}