fix: validation hardening — agent import prompt limit, relay retry tracking, heartbeat validation

- agent_import: add system_prompt length validation (max 50K chars)
  to prevent excessive token consumption from imported configs
- relay retry_task: wrap JoinHandle to log abort on server shutdown
- device_heartbeat: validate device_id length (1-64 chars) matching
  register endpoint constraints

desktop/src-tauri/src/kernel_commands/agent.rs

@@ -295,6 +295,18 @@ pub async fn agent_import(
     let mut config: AgentConfig = serde_json::from_str(&config_json)
         .map_err(|e| format!("Invalid agent config JSON: {}", e))?;
 
+    // Validate system_prompt length to prevent excessive token consumption
+    const MAX_SYSTEM_PROMPT_LEN: usize = 50_000;
+    if let Some(ref prompt) = config.system_prompt {
+        if prompt.len() > MAX_SYSTEM_PROMPT_LEN {
+            return Err(format!(
+                "system_prompt too long: {} chars (max {})",
+                prompt.len(),
+                MAX_SYSTEM_PROMPT_LEN
+            ));
+        }
+    }
+
     // Regenerate ID to avoid collisions
     config.id = AgentId::new();