refactor: rename diigo-dump to diigo-cache

Pure rename ahead of the dump worker's evolution into a bookmark cache
(goal 1c), so history follows the files. The diigo-login helper script
is dropped: the cookie now enters through webapp's mint form, never a
local script.

diigo-cache/package.json

@@ -0,0 +1,11 @@
+{
+	"name": "diigo-dump",
+	"private": true,
+	"scripts": {
+		"dev": "wrangler dev -c wrangler.jsonc -c ../webapp/wrangler.jsonc",
+		"diigo-login": "scripts/diigo-login"
+	},
+	"devDependencies": {
+		"wrangler": "^4.148.0"
+	}
+}