feat(core): enable shell background completion injection

With the idle wake-up listener in place, shell background
completions can now use 'inject' to feed output back into the
model conversation when the agent is idle.

packages/core/src/services/shellExecutionService.ts

@@ -455,7 +455,7 @@ export class ShellExecutionService {
                 : 'successfully';
               return `[Background command completed ${status}. Output saved to ${logPath}]`;
             },
-            completionBehavior: 'silent',
+            completionBehavior: 'inject',
           })
         : undefined;
 
@@ -797,7 +797,7 @@ export class ShellExecutionService {
             : 'successfully';
           return `[Background command completed ${status}. Output saved to ${logPath}]`;
         },
-        completionBehavior: 'silent',
+        completionBehavior: 'inject',
       }).result;
 
       let processingChain = Promise.resolve();