feat: scaffold codespace backend

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

internal/watcher/watcher.go

@@ -0,0 +1,15 @@
+package watcher
+
+// Event represents a filesystem change event.
+type Event struct {
+	Type string `json:"type"`
+	Path string `json:"path"`
+}
+
+// Watcher is a placeholder interface for future filesystem watching.
+// Not wired into the API in v1.
+type Watcher interface {
+	Start(root string) error
+	Stop() error
+	Events() <-chan Event
+}