[BUG] Cloud routine (CCR) stuck at "Claude Code process started" — never executes

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

On-demand "run now" triggers of a scheduled cloud routine (created via the /v1/code/triggers API, aka claude.ai routines) reliably provision a sandbox, log Claude Code process started, and then hang forever. No further log lines, no tool calls, no completion, no error. Reproduced 5/5 times across different configurations, ruling out anything in the routine's own config.

### What Should Happen?

On-demand "run now" triggers of a scheduled cloud routine (created via the /v1/code/triggers API, aka claude.ai routines) reliably provision a sandbox, log Claude Code process started, and then hang forever. No further log lines, no tool calls, no completion, no error. Reproduced 5/5 times across different configurations, ruling out anything in the routine's own config.

### Error Messages/Logs

```shell
Every attempt produces exactly this log, then stops:

env[info]: Allocating sandbox
env[info]: Environment runner started
env[info]: Initializing environment
env[info]: No sources configured
env[info]: No setup script configured
env[info]: Parallel initialization completed
env[info]: Claude Code executor created
env[info]: Starting Claude Code
env[info]: Claude Code process started
```

### Steps to Reproduce

Create a routine via POST /v1/code/triggers with job_config.ccr pointing at either available environment_id (env_01D3UrT2t3H8nKj5SS6r6XuD or env_013ebLpX1V4LWogeoDi7wiRA).
Trigger it via POST /v1/code/triggers/{id}/run, or click "Run" in the claude.ai routines UI.
Poll GET /v1/code/sessions/{session_id}/events.

### Claude Model

Sonnet (default)

### Is this a regression?

I don't know

### Last Working Version

_No response_

### Claude Code Version

Claude Code version 2.1.68.

### Platform

Anthropic API

### Operating System

macOS

### Terminal/Shell

Terminal.app (macOS)

### Additional Information

_No response_

Start by reproducing the run through POST /v1/code/triggers/{id}/run, then poll GET /v1/code/sessions/{session_id}/events and compare the reported log sequence. The fix is complete when a cloud routine proceeds beyond “Claude Code process started,” produces tool calls, and reaches completion without hanging.

Written by the indexing model from the issue text.