step 10: plugins system (Lua + templates) + DokuWiki as optional plugin

Plugin Manager:
- Discover plugins from .verstak/plugins/<name>/plugin.json
- Enable/disable per plugin
- Template definitions (JSON) → pre-filled node trees
- SQL migrations from plugins
- Built-in templates loaded from internal/core/plugins/builtin/templates/

Lua Runtime:
- Stub (gopher-lua placeholder) — ready for real implementation
- When dep added: hooks (on_init, on_vault_open, on_node_create),
  sandbox (no io/os.execute), Plugin API

GUI:
- Template selector in create node modal
- POST /api/nodes/from-template creates tree from template
- Built-in "Клиент" template: Overview note + Документы/Переписка/Скриншоты

CLI:
- verstak plugin list/enable/disable/templates

DokuWiki Importer:
- Moved to contrib/plugins/importer-dokuwiki/ (optional plugin)
- plugin.json + migration + README

DokuWiki removed from MVP core — now an opt-in plugin.

Acceptance: go build ./... pass, go test ./... pass (all packages).

internal/core/plugins/builtin/templates/client.json

@@ -0,0 +1,10 @@
+{
+  "name": "Клиент",
+  "root_type": "case",
+  "tree": [
+    { "type": "note", "title": "Overview" },
+    { "type": "folder", "title": "Документы" },
+    { "type": "folder", "title": "Переписка" },
+    { "type": "folder", "title": "Скриншоты" }
+  ]
+}