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/time-and-comic-to-ntfy.yaml

@@ -3,7 +3,7 @@ description: >
   Fan-in from two scripts (current time + monkeyuser comic), then send to ntfy.
 scripts:
   - id: time
-    script: get-current-time.js
+    script: plugin/get-current-time
 
   - id: comic
     script: fetch-html.js