refactor: Extract processExists to util.ProcessExists (gt-560ge)

Move duplicated processExists function to shared util package:
- Create internal/util/process.go with ProcessExists function
- Add internal/util/process_test.go with basic tests
- Update witness/manager.go to use util.ProcessExists
- Update refinery/manager.go to use util.ProcessExists
- Remove local processExists functions from both files

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/util/process.go

@@ -0,0 +1,13 @@
+package util
+
+import "os"
+
+// ProcessExists checks if a process with the given PID exists.
+// It uses the Unix convention of sending signal 0 to test for process existence.
+func ProcessExists(pid int) bool {
+	proc, err := os.FindProcess(pid)
+	if err != nil {
+		return false
+	}
+	return proc.Signal(nil) == nil
+}

internal/util/process_test.go

@@ -0,0 +1,25 @@
+package util
+
+import (
+	"testing"
+)
+
+func TestProcessExistsNonExistent(t *testing.T) {
+	// Using a very high PID that's unlikely to exist
+	pid := 999999999
+	if ProcessExists(pid) {
+		t.Errorf("ProcessExists(%d) = true, want false for non-existent process", pid)
+	}
+}
+
+func TestProcessExistsNegativePID(t *testing.T) {
+	// Negative PIDs are invalid and should return false or may cause errors
+	// depending on the platform, so just test that it doesn't panic
+	_ = ProcessExists(-1)
+}
+
+func TestProcessExistsZero(t *testing.T) {
+	// PID 0 is special (kernel process on Unix)
+	// Test that we can call it without panicking
+	_ = ProcessExists(0)
+}