cli: declare cross-spawn so vendored pi-claude-cli can resolve it

pi-claude-cli is bundled into the CLI's dist as a sibling tree at
/app/packages/cli/dist/pi-claude-cli, but its package.json (a
vendored copy) is informational only — pnpm doesn't link node_modules
beneath dist. The runtime extension loader does a Node ESM require
of cross-spawn from process-manager.ts and the resolver walks up to
/app/packages/cli/node_modules, where cross-spawn was not present
because no direct dependency in that package referenced it. Result
at startup: "Failed to load extension: Cannot find module
'cross-spawn'", which means the pi-claude-cli provider never
registered, which means triage's specifyTask path errored with "No
API provider registered for api: pi-claude-cli" the moment it tried
to plan.

Add cross-spawn as a direct dependency of the CLI package so pnpm
hoists it next to the vendored extension.

packages/cli/package.json

@@ -50,6 +50,7 @@
   "dependencies": {
     "@mariozechner/pi-ai": "^0.70.0",
     "@mariozechner/pi-coding-agent": "^0.70.0",
+    "cross-spawn": "^7.0.6",
     "express": "^5.1.0",
     "ink": "^6.8.0",
     "ink-spinner": "^5.0.0",