Feature request: per-sandbox credential injection configuration for Docker sandboxes

### Description

  The current credential injection system reads API credentials from host-level environment variables (e.g.,
  `GITHUB_TOKEN`, `ANTHROPIC_API_KEY`), making them global across all sandboxes. This doesn't support workflows
  where different projects need different credentials — for example, different GitHub tokens with different org
  scopes or permission levels per project.
