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/config.rs

@@ -1,11 +1,15 @@
 use serde::{Deserialize, Serialize};
 
+use crate::adapters::AdapterInstanceConfig;
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Config {
     pub server: ServerConfig,
     pub forgejo: ForgejoConfig,
     pub matrix: MatrixConfig,
     pub orchestrator: OrchestratorConfig,
+    #[serde(default)]
+    pub adapters: Vec<AdapterInstanceConfig>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -63,6 +67,7 @@ impl Default for Config {
                 task_timeout_secs: 1800,
                 default_max_retries: 2,
             },
+            adapters: vec![],
         }
     }
 }