Fix cooperative scheduling busy-wait issue

The wait_for_pid_exit function was spinning in a tight loop without yielding
control, preventing the context switch from actually happening. This fix
replaces the spin loop with yield_current() to allow the scheduler to run
the target process.

This is the critical fix for the CI syscall test failure.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ryanbreen

kernel/src/task/scheduler.rs

@@ -366,10 +366,9 @@ pub fn wait_for_pid_exit(pid: crate::process::ProcessId, timeout_ms: u64) -> Pro
             }
         }
 
-        // Small delay to prevent busy waiting
-        for _ in 0..100000 {
-            core::hint::spin_loop();
-        }
+        // Actually yield control to allow other processes to run
+        // This is the key fix - we need to yield so the context switch can happen
+        yield_current();
     }
 
     ProcessExitStatus::StillRunning