feat: implement serve mode with file watching

- Add fsnotify for file watching in serve mode
- Create watcher.go for content directory monitoring
- Update serve mode to generate initial output and watch for changes
- Add route to serve generated HTML files from output directory
- File changes trigger automatic regeneration

Co-Authored-By: Claude (glm-5) <noreply@anthropic.com>

watcher.go

@@ -0,0 +1,51 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/fsnotify/fsnotify"
+)
+
+// startWatcher starts watching the content directory for changes
+func startWatcher() (*fsnotify.Watcher, error) {
+	watcher, err := fsnotify.NewWatcher()
+	if err != nil {
+		return nil, fmt.Errorf("failed to create watcher: %w", err)
+	}
+
+	// Add content directory to watcher
+	err = watcher.Add(contentPath)
+	if err != nil {
+		watcher.Close()
+		return nil, fmt.Errorf("failed to watch content directory: %w", err)
+	}
+
+	fmt.Printf("Watching %s for changes...\n", contentPath)
+
+	// Start goroutine to handle events
+	go func() {
+		for {
+			select {
+			case event, ok := <-watcher.Events:
+				if !ok {
+					return
+				}
+				// Only process write and create events for .md files
+				if event.Has(fsnotify.Write) || event.Has(fsnotify.Create) {
+					fmt.Printf("File changed: %s\n", event.Name)
+					// Regenerate output
+					if err := generateOutput(); err != nil {
+						fmt.Printf("Error regenerating: %s\n", err)
+					}
+				}
+			case err, ok := <-watcher.Errors:
+				if !ok {
+					return
+				}
+				fmt.Printf("Watcher error: %s\n", err)
+			}
+		}
+	}()
+
+	return watcher, nil
+}