Add AWS Bedrock support to the Claude harness

## 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.

Start by locating the Claude harness and its existing environment-variable handling. Add recognition for CLAUDE_CODE_USE_BEDROCK, AWS_BEARER_TOKEN_BEDROCK, and AWS_REGION so requests use Bedrock when they are present, then verify the default Anthropic path remains unchanged.

Written by the indexing model from the issue text.