fix: add database migration for backend column and fix agent selection race condition

- Add run_migrations() to SQLite store that adds 'backend' column to existing
  missions tables (CREATE TABLE IF NOT EXISTS doesn't add columns to existing tables)
- Fix race condition in new-mission-dialog when switching backends: wait for
  backendAgents to finish loading before setting default agent, and only use
  fallback agents for opencode backend

dashboard/src/components/new-mission-dialog.tsx

@@ -65,7 +65,7 @@ export function NewMissionDialog({
   });
 
   // SWR: fetch agents for selected backend
-  const { data: backendAgents } = useSWR<BackendAgent[]>(
+  const { data: backendAgents, isLoading: backendAgentsLoading } = useSWR<BackendAgent[]>(
     newMissionBackend ? `backend-${newMissionBackend}-agents` : null,
     () => listBackendAgents(newMissionBackend),
     { revalidateOnFocus: false, dedupingInterval: 30000 }
@@ -81,8 +81,11 @@ export function NewMissionDialog({
     dedupingInterval: 30000,
   });
 
-  // Parse agents from either backend API or fallback
-  const agents = backendAgents?.map(a => a.name) || parseAgentNames(agentsPayload);
+  // Parse agents from backend API (only use fallback for opencode backend)
+  // For non-opencode backends, wait for backendAgents to load to avoid race condition
+  const agents = newMissionBackend === 'opencode'
+    ? (backendAgents?.map(a => a.name) || parseAgentNames(agentsPayload))
+    : (backendAgents?.map(a => a.name) || []);
 
   const formatWorkspaceType = (type: Workspace['workspace_type']) =>
     type === 'host' ? 'host' : 'isolated';
@@ -104,9 +107,13 @@ export function NewMissionDialog({
   // Set default agent when dialog opens (only once per open)
   // Wait for both agents AND config to load before setting defaults
   useEffect(() => {
-    if (!open || defaultSet || agents.length === 0) return;
+    if (!open || defaultSet) return;
     // Wait for config to finish loading (undefined = still loading, null/object = loaded)
     if (config === undefined) return;
+    // Wait for backend agents to finish loading (avoid race condition when switching backends)
+    if (backendAgentsLoading) return;
+    // If no agents available yet, wait
+    if (agents.length === 0) return;
 
     if (config?.default_agent && agents.includes(config.default_agent)) {
       setNewMissionAgent(config.default_agent);
@@ -114,7 +121,7 @@ export function NewMissionDialog({
       setNewMissionAgent('Sisyphus');
     }
     setDefaultSet(true);
-  }, [open, defaultSet, agents, config]);
+  }, [open, defaultSet, agents, config, backendAgentsLoading]);
 
   const resetForm = () => {
     setNewMissionWorkspace('');

src/api/mission_store/sqlite.rs

@@ -106,6 +106,9 @@ impl SqliteMissionStore {
             conn.execute_batch(SCHEMA)
                 .map_err(|e| format!("Failed to run schema: {}", e))?;
 
+            // Run migrations for existing databases
+            Self::run_migrations(&conn)?;
+
             Ok::<_, String>(conn)
         })
         .await
@@ -155,6 +158,29 @@ impl SqliteMissionStore {
             String::new()
         }
     }
+
+    /// Run database migrations for existing databases.
+    /// CREATE TABLE IF NOT EXISTS doesn't add columns to existing tables,
+    /// so we need to handle schema changes manually.
+    fn run_migrations(conn: &Connection) -> Result<(), String> {
+        // Check if 'backend' column exists in missions table
+        let has_backend_column: bool = conn
+            .prepare("SELECT 1 FROM pragma_table_info('missions') WHERE name = 'backend'")
+            .map_err(|e| format!("Failed to check for backend column: {}", e))?
+            .exists([])
+            .map_err(|e| format!("Failed to query table info: {}", e))?;
+
+        if !has_backend_column {
+            tracing::info!("Running migration: adding 'backend' column to missions table");
+            conn.execute(
+                "ALTER TABLE missions ADD COLUMN backend TEXT NOT NULL DEFAULT 'opencode'",
+                [],
+            )
+            .map_err(|e| format!("Failed to add backend column: {}", e))?;
+        }
+
+        Ok(())
+    }
 }
 
 fn parse_status(s: &str) -> MissionStatus {