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/worker/src/session-manager.ts

@@ -103,7 +103,6 @@ export const AGENTS: Readonly<Record<AgentName, AgentDefinition>> = Object.freez
     prerequisites: ['injection-exploit', 'xss-exploit', 'auth-exploit', 'ssrf-exploit', 'authz-exploit'],
     promptTemplate: 'report-executive',
     deliverableFilename: 'comprehensive_security_assessment_report.md',
-    modelTier: 'small',
   },
 });