feat: Add gt rig boot command to start witness and refinery

Inverse of 'gt rig shutdown'. Starts rig patrol agents:
- Checks tmux sessions to avoid duplicates
- Starts witness if not running
- Starts refinery if not running
- Reports what was started vs skipped

Also adds ProcessExists util function needed by witness/refinery managers.

internal/util/process.go

@@ -1,3 +1,24 @@
 // Package util provides utility functions for Gas Town.
 // This file was created as part of an E2E polecat workflow test.
 package util
+
+import (
+	"os"
+	"syscall"
+)
+
+// ProcessExists checks if a process with the given PID exists.
+// It sends signal 0 to the process, which doesn't actually send a signal
+// but does perform error checking to see if the process exists.
+func ProcessExists(pid int) bool {
+	if pid <= 0 {
+		return false
+	}
+	process, err := os.FindProcess(pid)
+	if err != nil {
+		return false
+	}
+	// Signal 0 checks if process exists without sending a real signal
+	err = process.Signal(syscall.Signal(0))
+	return err == nil
+}