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>

apps/cli/src/config/writer.ts

@@ -13,7 +13,6 @@ export interface ShannonConfig {
   custom_base_url?: { base_url?: string; auth_token?: string };
   bedrock?: { use?: boolean; region?: string; token?: string };
   vertex?: { use?: boolean; region?: string; project_id?: string; key_path?: string };
-  router?: { default?: string; openai_key?: string; openrouter_key?: string };
   models?: { small?: string; medium?: string; large?: string };
 }