feat(connectd): companion sidecar lifecycle (connectd-child, v1)

Adds optional ServerConfig.companion {command,args,env,healthUrl}. On stdio connect, ensureCompanion
spawns the sidecar (e.g. the WhatsApp bridge) if healthUrl isn't already serving, polls health (15s),
then spawns the MCP server that talks to it. Killed on closeServer + SIGINT/SIGTERM (disposeCompanions)
so it never orphans. Reuses an already-running bridge (idempotent). Typecheck clean.

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

src/config.ts

@@ -11,6 +11,16 @@ export interface ServerConfig {
   command?: string;
   args?: string[];
   env?: Record<string, string>;
+  // Companion sidecar: a long-running process this connector's MCP server talks to (e.g. the
+  // WhatsApp bridge on :8080 that holds the paired session). connectd starts it as a CHILD when the
+  // connector connects, waits for healthUrl to respond, then spawns the MCP server; it is killed on
+  // disconnect/shutdown. `command` supports @bin//@node//@bundled resolution like the server itself.
+  companion?: {
+    command: string;
+    args?: string[];
+    env?: Record<string, string>;
+    healthUrl?: string;
+  };
   // http (Phase 3)
   url?: string;
   auth?: "none" | "oauth" | "token" | "google";