fix(update-check): redirect install script stdout to stderr in --output json mode (#2919)

When --output json is requested, the auto-update install script was
running with stdio: "inherit", causing [spawn] install messages to
pollute stdout before the JSON result, breaking JSON consumers.

Fix:
- Pre-scan process.argv for --output json before checkForUpdates()
  is called in index.ts (formal flag parsing happens later at line 944)
- Pass jsonOutput flag through checkForUpdates() -> performAutoUpdate()
- When jsonOutput=true, use stdio: ["pipe", stderr, stderr] for the
  install script execution so all output goes to stderr only
- Set SPAWN_CLI_UPDATED=1 env var on re-exec so JSON consumers can
  detect the update via cli_updated: true in SpawnResult
- Add cli_updated?: boolean to SpawnResult interface in commands/run.ts
- Add tests covering both json and non-json stdio behavior

Fixes #2918

Agent: issue-fixer

Co-authored-by: B <6723574+louisgv@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

packages/cli/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@openrouter/spawn",
-  "version": "0.25.21",
+  "version": "0.25.22",
   "type": "module",
   "bin": {
     "spawn": "cli.js"