steps 4-6 + doc overhaul: files, notes, GUI, plugins docs

DOCUMENTATION (shift from personal to universal product):
- README.md: rewritten with 'one product, different doors' framing,
  universal entities table, audience segments
- 01_Product_Spec.md: removed personal references (sshkeeper, Godot,
  DokuWiki, servers), added audience segments (freelancer, repairmaster,
  developer, maker, consultant), universal scenarios
- 02_Architecture.md: added 'Plugins (Extensibility)' section with
  calendar/kanban/importer/template examples
- 03_Data_Model_Storage.md: added section 6 on plugin extensibility
  (node_meta, type registry, SQL migrations per plugin)
- 09_Extensibility.md (NEW): full plugin architecture — Lua runtime,
  plugin.json, hooks, sandbox, templates, registry
- PLAN.md: added step 16 (plugins), updated status table
- 00_README.md: rewritten product index with plugin principle

CODE — STEP 4 (Files):
- migration 002: files table (id, node_id, filename, path,
  storage_mode, size, sha256, mime, ...)
- FileService: AddExternal, CopyIntoVault, Get, ListByNode,
  MarkMissing, DeleteToTrash, Open (xdg-open)
- file_test.go: 5 tests (external, copy-vault, list-node,
  delete-trash, MIME guess)

CODE — STEP 5 (Notes):
- migration 003: notes table (node_id PK, file_id, format,
  original_format, encrypted)
- NoteService: Create (node+file+link), Read, Save (with backup to
  .verstak/history/), Delete, Load
- note_test.go: 3 tests (create-read, save-backup, delete)

CODE — STEP 6 (GUI):
- cmd/verstak-gui/main.go: launches GUI server, opens browser
- internal/gui/server.go: HTTP API for nodes/notes/files/search
- internal/gui/index.html.go: full inline SPA frontend (dark theme,
  sidebar tree, cards grid, note editor, search, create modals)
- Navigation: sidebar tree → click node → detail view with
  children + files cards → tab switch (overview/notes/files)
  → create node/note via modal → edit note in fullscreen
  textarea → save (with history backup)

Acceptance: go build ./... pass, go build -tags gui ./cmd/verstak-gui pass,
  go test ./... pass (20+ tests). GUI serves on random port, opens browser.
  API returns JSON for all resource types.

cmd/verstak-gui/main.go

@@ -0,0 +1,71 @@
+//go:build gui
+// +build gui
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"os/exec"
+	"os/signal"
+	"path/filepath"
+	"runtime"
+	"syscall"
+
+	gui "verstak/internal/gui"
+	"verstak/internal/core/storage"
+)
+
+func main() {
+	vaultPath := "."
+	if len(os.Args) > 1 {
+		vaultPath = os.Args[1]
+	}
+
+	abs, err := filepath.Abs(vaultPath)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	dbPath := filepath.Join(abs, ".verstak", "index.db")
+	db, err := storage.Open(dbPath)
+	if err != nil {
+		log.Fatalf("Open vault: %v", err)
+	}
+	defer db.Close()
+
+	srv := gui.NewServer(db, abs)
+	addr, err := srv.Start()
+	if err != nil {
+		log.Fatalf("Start GUI: %v", err)
+	}
+
+	fmt.Println("Верстак GUI:", addr)
+	openBrowser(addr)
+
+	// Wait for interrupt.
+	sig := make(chan os.Signal, 1)
+	signal.Notify(sig, syscall.SIGINT, syscall.SIGTERM)
+	<-sig
+
+	srv.Stop()
+	deferFunc()
+}
+
+func openBrowser(url string) {
+	var cmd *exec.Cmd
+	switch runtime.GOOS {
+	case "linux":
+		cmd = exec.Command("xdg-open", url)
+	case "darwin":
+		cmd = exec.Command("open", url)
+	case "windows":
+		cmd = exec.Command("cmd", "/c", "start", "", url)
+	}
+	if cmd != nil {
+		go cmd.Start()
+	}
+}
+
+func deferFunc() {}