feat(plugins): core read-only scripts and user plugin system

Introduce plugin/<id> installs (zip, HTTPS git, example, fork),
jerapah-plugin.json manifests with jerapah semver ranges, isolated
plugin dirs under data/plugins, and app version 0.1.0. Core scripts
are non-editable; get-current-time moves to examples/plugins.

Co-authored-by: Nasyarobby Putra <nasyarobby@gmail.com>

examples/plugins/get-current-time/package.json

@@ -0,0 +1,7 @@
+{
+  "name": "jflow-plugin-get-current-time",
+  "version": "0.1.0",
+  "private": true,
+  "type": "module",
+  "description": "Example JerapahFlow plugin (no npm dependencies)"
+}