fix(cli): validate agentId in fn_task_create/update and unblock bundle tests

fn_task_create and fn_task_update accepted any string as `agentId` and
wrote it verbatim onto `task.assignedAgentId`, letting hallucinated IDs
(e.g. `agent-executor-001`) appear as agent badges in the dashboard.
Mirror the validation already used by fn_delegate: look the agent up via
AgentStore and reject unknown or ephemeral/runtime-managed agents. Null
still clears the field on update.

Also clean up two stale failures in bundle-output.test that predated this
change:
- pi-claude-cli no longer imports cross-spawn, so drop the dependency and
  its orphan type-decl file.
- Loosen the spawn-import regex to match `spawn` anywhere in the
  destructured import (the source has additional named imports).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/pi-claude-cli/src/types/cross-spawn.d.ts

@@ -1,7 +0,0 @@
-declare module "cross-spawn" {
-  const spawn: typeof import("node:child_process").spawn & {
-    sync: typeof import("node:child_process").spawnSync;
-  };
-
-  export default spawn;
-}