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).

contrib/plugins/importer-dokuwiki/README.md

@@ -0,0 +1,16 @@
+# DokuWiki Importer
+
+Import your DokuWiki pages and namespaces into Verstak.
+
+Place this directory in `.verstak/plugins/importer-dokuwiki/` inside your vault,
+then run `verstak plugin enable importer-dokuwiki`.
+
+## Usage
+
+    verstak import-dokuwiki --pages /path/to/data/pages --media /path/to/data/media
+
+## What it does
+
+- namespaces → nodes tree
+- pages → md notes inside the tree
+- originals saved in `.verstak/originals/dokuwiki/`

contrib/plugins/importer-dokuwiki/migrations/001_create_originals.sql

@@ -0,0 +1,7 @@
+CREATE TABLE IF NOT EXISTS dokuwiki_originals (
+    id TEXT PRIMARY KEY,
+    node_id TEXT NOT NULL REFERENCES nodes(id),
+    source_path TEXT NULL,
+    source_type TEXT NOT NULL,
+    imported_at TEXT NOT NULL
+);

contrib/plugins/importer-dokuwiki/plugin.json

@@ -0,0 +1,11 @@
+{
+  "name": "importer-dokuwiki",
+  "version": "0.1.0",
+  "description": "Import pages and media from DokuWiki into Verstak",
+  "author": "Verstak contributors",
+  "hooks": {
+    "on_init": "on_init"
+  },
+  "node_types": ["dokuwiki_page", "dokuwiki_ns"],
+  "migrations": ["001_create_originals.sql"]
+}