fix: prevent sprite idle shutdown during agent install (#2874)

The sprite was going idle and shutting down during long npm install
operations because the remote keep-alive script wasn't installed yet
and sprite exec alone doesn't count as activity.

- Add local keep-alive that pings the sprite's public URL every 30s
  from the client machine during provisioning and agent install
- Stop it when the interactive session starts (remote script takes over)
- Add i/o timeout to spriteRetry's transient error regex so connection
  timeouts are retried instead of failing immediately

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/cli/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@openrouter/spawn",
-  "version": "0.25.12",
+  "version": "0.25.13",
   "type": "module",
   "bin": {
     "spawn": "cli.js"