feat: Agent Adapter Interface (Task 7)

- AgentAdapter trait: register, heartbeat, execute, submit_receipt, deregister
- AdapterRunner: lifecycle management (start with health check, heartbeat loop, graceful stop)
- AdapterInstanceConfig: per-adapter config (type, work_dir, model, capabilities, env, connection)
- Config integration: adapters field in Config + config.example.toml
- 3 tests: config extraction, runner lifecycle, fake execute

22/22 tests pass.

src/main.rs

@@ -1,3 +1,4 @@
+mod adapters;
 mod api;
 mod config;
 mod core;