fix: validate agents exist before dev/deploy commands

Fixes #150 and #151 where CLI crashes or fails late when no agents
are defined in the project. Now shows helpful error messages instead.

- DevScreen: Add noAgentsError state and render error screen with
  guidance to run 'agentcore add agent'
- preflight: Validate agents array in validateProject() before
  proceeding with deployment

Author: tejaskash

src/cli/operations/deploy/preflight.ts

@@ -70,6 +70,13 @@ export async function validateProject(): Promise<PreflightContext> {
   const projectSpec = await configIO.readProjectSpec();
   const awsTargets = await configIO.readAWSDeploymentTargets();
 
+  // Validate that at least one agent is defined
+  if (!projectSpec.agents || projectSpec.agents.length === 0) {
+    throw new Error(
+      'No agents defined in project. Add at least one agent with "agentcore add agent" before deploying.'
+    );
+  }
+
   // Validate runtime names don't exceed AWS limits
   validateRuntimeNames(projectSpec);
 

src/cli/tui/screens/dev/DevScreen.tsx

@@ -111,6 +111,7 @@ export function DevScreen(props: DevScreenProps) {
   const [selectedAgentIndex, setSelectedAgentIndex] = useState(0);
   const [selectedAgentName, setSelectedAgentName] = useState<string | undefined>(props.agentName);
   const [agentsLoaded, setAgentsLoaded] = useState(false);
+  const [noAgentsError, setNoAgentsError] = useState(false);
 
   const workingDir = props.workingDir ?? process.cwd();
 
@@ -136,8 +137,8 @@ export function DevScreen(props: DevScreenProps) {
         setSelectedAgentName(agents[0].name);
         setMode('input');
       } else if (agents.length === 0) {
-        // No supported agents, will show error via useDevServer
-        setMode('input');
+        // No supported agents, show error screen
+        setNoAgentsError(true);
       }
 
       setAgentsLoaded(true);
@@ -323,10 +324,25 @@ export function DevScreen(props: DevScreenProps) {
   );
 
   // Return null while loading
-  if (!agentsLoaded || (mode !== 'select-agent' && (!configLoaded || !config))) {
+  if (!agentsLoaded || (mode !== 'select-agent' && !noAgentsError && (!configLoaded || !config))) {
     return null;
   }
 
+  // Show error screen if no agents are defined
+  if (noAgentsError) {
+    return (
+      <Screen title="Dev Server" onExit={props.onBack} helpText="Esc quit">
+        <Box flexDirection="column">
+          <Text color="red">No agents defined in project.</Text>
+          <Text>Dev mode requires at least one Python agent with an entrypoint.</Text>
+          <Text>
+            Run <Text color="blue">agentcore add agent</Text> to create one.
+          </Text>
+        </Box>
+      </Screen>
+    );
+  }
+
   const statusColor = { starting: 'yellow', running: 'green', error: 'red', stopped: 'gray' }[status];
 
   // Visible lines for display