whatsapp: fix history sync + session corruption, Baileys 7.0.0-rc14 (v7 / 2.1.0)

Diagnosed from the live server (two users, two accounts, one shared log).

- history sync was gated off entirely on Baileys 6.7.x: the library derives
  shouldSyncHistoryMessage from syncFullHistory when unset, and the connector
  shipped syncFullHistory: false. Now both are passed explicitly, so behaviour
  no longer depends on the resolved Baileys version. browser -> ['Mac OS', ...]
  because PLATFORM_MAP only grants a desktop-grade sync to Mac OS / Windows.
- reconnects leaked their socket, leaving two writers over auth/: no end(), no
  removeAllListeners(). ~15 reconnects/day produced 1422 Bad MAC lines, all on
  the account's own LID device 0. Added teardownSock(), per-socket generation
  guards, and a single-slot reconnect timer with exponential backoff.
- undecryptable messages (stubType CIPHERTEXT) were stored with empty text and
  rendered as blank transcript lines; now labelled and counted. Text-less
  protocol frames are dropped.
- fetchLatestBaileysVersion() ran on every reconnect; now cached for 6h.
- libsignal's direct console.error spam collapsed into one counted line, log
  lines tagged with the linked account, console.log redirected off stdout.

Baileys pinned ^6.7.9 -> 7.0.0-rc14 (the range resolved to 6.7.24 for one user
and 6.17.16 for the other). RC is ESM-only, so index.js is now ESM.

Verified on skald-runtime:v4 (Node 20.19.2): initialize, tools/list, status,
list_chats, version fetch, QR, and logout -> teardown -> single reconnect.

Remaining gap: the store is still in-memory only, so synced history is lost on
the next process restart.

connectors/whatsapp/package.json

@@ -1,14 +1,15 @@
 {
   "name": "skald-whatsapp-mcp",
-  "version": "2.0.0",
+  "version": "2.1.0",
   "private": true,
+  "type": "module",
   "description": "WhatsApp MCP connector for Skald (Baileys, no browser).",
   "main": "index.js",
   "engines": {
-    "node": ">=18"
+    "node": ">=20"
   },
   "dependencies": {
-    "@whiskeysockets/baileys": "^6.7.9",
-    "qrcode": "^1.5.4"
+    "@whiskeysockets/baileys": "7.0.0-rc14",
+    "qrcode": "1.5.4"
   }
 }