feat(connectd): companion cwd (writable session dir for the WhatsApp bridge)

companion.cwd (created if missing; ~/$HOME expanded) sets the sidecar's working directory so the
WhatsApp bridge writes its session store to a writable path (~/.neuron/whatsapp), never the read-only
app bundle. Typecheck clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/config.ts

@@ -20,6 +20,10 @@ export interface ServerConfig {
     args?: string[];
     env?: Record<string, string>;
     healthUrl?: string;
+    // Working directory for the sidecar (created if missing). The WhatsApp bridge writes its session
+    // store relative to cwd, so this must be writable (e.g. ~/.neuron/whatsapp) — never the read-only
+    // app bundle. "~" / "$HOME" at the start are expanded to the user's home.
+    cwd?: string;
   };
   // http (Phase 3)
   url?: string;