fix: Pre-seed Claude Code config to fix silent auth failure in sandbox

[skinnerjc]

Problem

When running Claude Code 2.1.37 (current stable) in --print mode via the sandbox agent bridge, Claude Code exits with rc=0 and empty stdout/stderr — no API calls are made through the bridge. With attempts set to unlimited, the retry loop spins for dozens of iterations.

What was observed

1. The bridge proxy works: a raw curl POST /v1/messages from inside the container returns a valid Anthropic API response.
2. The Claude Code binary works: claude --version and claude --help both succeed.
3. claude --print -- "Say hello" exits in <1 second with rc=0 and zero output, despite ANTHROPIC_AUTH_TOKEN being set in the environment.
4. Claude Code's debug log (~/.claude/debug/latest) shows:

   [API:request] Creating client, ANTHROPIC_CUSTOM_HEADERS present: false, has Authorization header: false
   [API:auth] OAuth token check starting
   

   The log ends immediately after — no API call is made.

Change

Before running Claude Code, write ~/.claude/settings.json with an apiKeyHelper that echoes the dummy API key from ANTHROPIC_AUTH_TOKEN. Claude Code then authenticates and makes API calls through the bridge on the first attempt.

Tested with Claude Code 2.1.37 on K8s sandbox (non-centaur path).