feat: wire agent.delegate tool with sub-agent configs

- Export createAgentDelegateTool through builtin/index.ts → tools/index.ts
- Register agent.delegate in routing.ts with lazy orchestrator pattern
- Add agent.delegate + agents.list to messaging and coding policy profiles
- Add group:agents tool group to policy.ts
- Add research/code/comms agent config examples to default.yaml
- Add research/code/comms agent configs to user config.yaml
- Add 11 tests for agent-delegate tool (all pass)
- Typecheck clean, no regressions

src/backends/index.ts

@@ -13,3 +13,13 @@ export {
   CLASSIFICATION_PROMPT,
   TOOL_SUMMARISATION_PROMPT,
 } from './native/index.js';
+export {
+  ExternalCliBackend,
+  ClaudeCodeBackend,
+  OpenCodeBackend,
+  CodexBackend,
+  GeminiBackend,
+  type ExternalBackend,
+  type ExternalBackendName,
+  type ExternalBackendRequest,
+} from './external.js';