fix(dev): prevent orphaned servers and handle empty prompt

Fixes found during bugbash of auto-start feature:

- Register process.on('exit') handler to kill auto-started server,
  ensuring cleanup even when invoke helpers call process.exit(1)
- Initialize resolveServerExit with safe default to prevent
  potential undefined call if Promise executor changes
- Add unref() to SIGKILL timer in DevServer.kill() so Node.js
  exits promptly instead of hanging 2 seconds
- Use !== undefined check for prompt so empty string "" enters
  invoke path instead of falling through to interactive TUI

Constraint: process.on('exit') handlers are synchronous; cannot await server shutdown
Rejected: Modify invoke helpers to throw instead of process.exit | too many callers to change
Confidence: high
Scope-risk: narrow
Not-tested: rapid consecutive invocations (port TIME_WAIT race)

Author: aidandaly24

src/cli/commands/dev/command.tsx

@@ -163,7 +163,7 @@ export const registerDev = (program: Command) => {
 
         // If a prompt is provided, invoke the dev server (auto-starting if needed)
         const invokePrompt = positionalPrompt;
-        if (invokePrompt) {
+        if (invokePrompt !== undefined) {
           const workingDir = getWorkingDirectory();
           const invokeProject = await loadProjectConfig(workingDir);
 
@@ -211,7 +211,8 @@ export const registerDev = (program: Command) => {
             }
 
             const serverErrors: string[] = [];
-            let resolveServerExit: () => void;
+            // eslint-disable-next-line @typescript-eslint/no-empty-function
+            let resolveServerExit: () => void = () => {};
             const serverExitPromise = new Promise<void>(resolve => {
               resolveServerExit = resolve;
             });
@@ -232,6 +233,10 @@ export const registerDev = (program: Command) => {
             });
             await server.start();
 
+            // Ensure server cleanup on any exit path (process.exit in invoke helpers, SIGINT, etc.)
+            const cleanupServer = () => server.kill();
+            process.on('exit', cleanupServer);
+
             // Wait for server to accept connections, bail early if process crashes
             const ready = await Promise.race([waitForServerReady(invokePort), serverExitPromise.then(() => false)]);
 
@@ -240,7 +245,6 @@ export const registerDev = (program: Command) => {
                 console.error(serverErrors.slice(-5).join('\n'));
               }
               console.error('Error: Dev server failed to start. Run "agentcore dev --logs" for details.');
-              server.kill();
               process.exit(1);
             }
             autoStartedServer = server;

src/cli/operations/dev/dev-server.ts

@@ -82,9 +82,10 @@ export abstract class DevServer {
   kill(): void {
     if (!this.child || this.child.killed) return;
     this.child.kill('SIGTERM');
-    setTimeout(() => {
+    const killTimer = setTimeout(() => {
       if (this.child && !this.child.killed) this.child.kill('SIGKILL');
     }, 2000);
+    killTimer.unref();
   }
 
   /** Mode-specific setup (e.g., venv creation, container image build). Returns false to abort. */