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>

packages/server/workflows/default/cron-example.yaml

@@ -2,7 +2,7 @@ name: cron example
 description: |
   this workflow triggered by cron
 scripts:
-  - script: get-current-time.js
+  - script: plugin/get-current-time
   - set:
       expression: '{"message": context.datetime}'
   - script: ntfy.js