backport: provider extensions and drop claude-code-router mode

Cherry-pick of KeygraphHQ/shannon#295 (581c208).

Upstream changes: removes router mode from CLI/worker, adds provider
extensions, new report-output-provider and checkpoint-provider interfaces,
refactored workflow orchestration.

Conflicts resolved: kept our README.md, CLAUDE.md, and deleted compose files.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -120,8 +120,6 @@ body:
         - "Custom base URL (proxy/gateway)"
         - "AWS Bedrock"
         - "Google Vertex AI"
-        - "Router - OpenAI"
-        - "Router - OpenRouter"
     validations:
       required: true