[FEATURE] Add interpolation for ANTHROPIC_CUSTOM_HEADERS

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

I'm using a sandbox that replaces placeholder text with a secret. It works for the auth token but doesn't for custom headers because the environment variable needs to be "key: value".

Proposed Solution

If custom headers support interpolation, I could define the header as "key: env:SPECIAL_VAR" and then set the secret for SPECIAL_VAR.

Codex supports this right now through their env_http_headers and I'd like to do the same with Claude Code.

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

Configuration and settings

Use Case Example

_No response_

Additional Context

_No response_