fix(backend): only kill processes started by TUI

Track PIDs of backends started by /backend command and only kill those
specific PIDs. Previous implementation used pkill which would kill all
Ollama/llama-server processes including those started by the user or
systemd services. Now we only terminate processes we started.

src/frontends/tui/minimal.ts

@@ -52,6 +52,7 @@ export class MinimalTui {
   private totalUsage: TokenUsage = { inputTokens: 0, outputTokens: 0 };
   private currentHint = '';
   private lastLine = '';
+  private backendPids: Map<string, number> = new Map();
 
   constructor(private config: MinimalTuiConfig) {}
 
@@ -322,24 +323,30 @@ export class MinimalTui {
   }
 
   private async stopBackend(provider: string): Promise<void> {
+    const pid = this.backendPids.get(provider);
+    if (!pid) {
+      return; // No tracked PID, process not started by us
+    }
+
     try {
-      const { spawn } = await import('child_process');
-      let processName: string;
-      switch (provider) {
-        case 'ollama':
-          processName = 'ollama';
-          break;
-        case 'llamacpp':
-          processName = 'llama-server';
-          break;
-        default:
-          return;
-      }
+      process.kill(pid, 'SIGTERM');
+      // Wait up to 2 seconds for graceful shutdown
       await new Promise<void>((resolve) => {
-        spawn('pkill', [processName]).on('close', resolve);
+        const timeout = setTimeout(resolve, 2000);
+        const checkInterval = setInterval(() => {
+          try {
+            process.kill(pid, 0); // Check if process exists
+          } catch {
+            clearInterval(checkInterval);
+            clearTimeout(timeout);
+            resolve();
+          }
+        }, 100);
       });
+      this.backendPids.delete(provider);
     } catch (error) {
-      // Ignore errors stopping backends
+      // Process already dead or permission error
+      this.backendPids.delete(provider);
     }
   }
 
@@ -348,18 +355,25 @@ export class MinimalTui {
       const { spawn } = await import('child_process');
       const args: string[] = [];
 
+      let proc: ReturnType<typeof spawn> | undefined;
+
       switch (provider) {
         case 'ollama':
-          spawn('ollama', ['serve'], { detached: true, stdio: 'ignore' }).unref();
+          proc = spawn('ollama', ['serve'], { detached: true, stdio: 'ignore' });
           break;
         case 'llamacpp':
           args.push('--model', config.model);
           args.push('--port', new URL(config.endpoint ?? 'http://localhost:8080').port || '8080');
           args.push('--host', '0.0.0.0');
-          spawn('llama-server', args, { detached: true, stdio: 'ignore' }).unref();
+          proc = spawn('llama-server', args, { detached: true, stdio: 'ignore' });
           break;
       }
 
+      if (proc && proc.pid) {
+        proc.unref();
+        this.backendPids.set(provider, proc.pid);
+      }
+
       // Wait briefly for the daemon to start
       await new Promise(resolve => setTimeout(resolve, 500));
     } catch (error) {