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
+}