feat: add persistent data volume for challenge containers

Mount /tmp/platform-challenges/{container}/data to /data in challenge containers.
This allows challenges to persist state (evaluation progress, etc.) across:
- Container restarts
- Image repulls
- Validator restarts

Each challenge+validator combination gets its own persistent directory.

Author: echobt

crates/challenge-orchestrator/src/docker.rs

@@ -301,6 +301,12 @@ impl DockerClient {
             }]),
         );
 
+        // Create persistent data directory for challenge state (survives restarts)
+        let challenge_data_dir = format!("/tmp/platform-challenges/{}/data", container_name);
+        if let Err(e) = std::fs::create_dir_all(&challenge_data_dir) {
+            warn!("Failed to create challenge data dir {}: {}", challenge_data_dir, e);
+        }
+
         // Build host config with resource limits
         let mut host_config = HostConfig {
             network_mode: Some(self.network_name.clone()),
@@ -309,10 +315,12 @@ impl DockerClient {
             memory: Some((config.memory_mb * 1024 * 1024) as i64),
             // Mount Docker socket for challenge containers to run agent evaluations
             // Mount tasks directory both to internal path AND to host path for Docker-in-Docker
+            // Mount persistent data directory for challenge state (evaluation progress, etc.)
             binds: Some(vec![
                 "/var/run/docker.sock:/var/run/docker.sock:rw".to_string(),
                 "/tmp/platform-tasks:/app/data/tasks:rw".to_string(), // Override internal tasks
                 "/tmp/platform-tasks:/tmp/platform-tasks:rw".to_string(), // For DinD path mapping
+                format!("{}:/data:rw", challenge_data_dir), // Persistent challenge state
             ]),
             ..Default::default()
         };