Add AWS Bedrock support to the Claude harness #247

Problem / Motivation

Scion currently has no support for AWS Bedrock when using the Claude harness. Users who access Claude via Bedrock API tokens cannot use Scion, as the Claude harness has no awareness of Bedrock's required environment variables.

Proposed Solution

Add Bedrock support to the Claude harness by recognizing the following environment variables when present:

CLAUDE_CODE_USE_BEDROCK
AWS_BEARER_TOKEN_BEDROCK
AWS_REGION

When these variables are detected, the Claude harness should route requests through Bedrock rather than the default Anthropic API endpoint, matching the default behavior of Claude CLI

Alternatives Considered

Proxying Bedrock traffic through a compatibility layer to make it appear as a standard Anthropic API endpoint is theoretically possible but adds unnecessary complexity and defeats the purpose of native Bedrock integration.

Additional Context

Scope is intentionally limited to API token authentication for the Claude harness only. Other auth methods and other Bedrock-hosted models could be addressed in follow-up issues if there is interest.
This would be particularly valuable for enterprise users who access Claude exclusively through Bedrock for compliance or cost reasons.