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.

internal/core/storage/migrations_002.sql.go

@@ -0,0 +1,21 @@
+package storage
+
+// migration002 — files table for vault file tracking.
+const migration002 = `
+CREATE TABLE IF NOT EXISTS files (
+    id TEXT PRIMARY KEY,
+    node_id TEXT NOT NULL REFERENCES nodes(id),
+    filename TEXT NOT NULL,
+    path TEXT NOT NULL,
+    storage_mode TEXT NOT NULL DEFAULT 'vault',
+    size INTEGER NOT NULL DEFAULT 0,
+    sha256 TEXT NULL,
+    mime TEXT NULL,
+    created_at TEXT NOT NULL,
+    updated_at TEXT NOT NULL,
+    last_seen_at TEXT NULL,
+    missing INTEGER NOT NULL DEFAULT 0
+);
+
+CREATE INDEX IF NOT EXISTS idx_files_node ON files(node_id);
+`

internal/core/storage/migrations_003.sql.go

@@ -0,0 +1,12 @@
+package storage
+
+// migration003 — notes table.
+const migration003 = `
+CREATE TABLE IF NOT EXISTS notes (
+    node_id TEXT PRIMARY KEY REFERENCES nodes(id),
+    file_id TEXT NOT NULL REFERENCES files(id),
+    format TEXT NOT NULL DEFAULT 'markdown',
+    original_format TEXT NULL,
+    encrypted INTEGER NOT NULL DEFAULT 0
+);
+`

internal/core/storage/storage.go

@@ -58,7 +58,9 @@ CREATE TABLE IF NOT EXISTS _schema_ver (
 
 var migrationFiles = map[int]string{
 	1: migration001,
-	// 2: migration002, etc.
+	2: migration002,
+	3: migration003,
+	// 4: migration004, etc.
 }
 
 func (db *DB) runInitialSchema() error {