art: Refactor agent code

Move out cmd pieces and split into its own package. Also make sure we reload
plugins before bootstrapping nodes.

Also commons out the agent configuration in a specific YAML file.

internal/agent/recovery_linux.go

@@ -0,0 +1,12 @@
+package agent
+
+import (
+	"os"
+	"syscall"
+	"unsafe"
+)
+
+func setWinsize(f *os.File, w, h int) {
+	syscall.Syscall(syscall.SYS_IOCTL, f.Fd(), uintptr(syscall.TIOCSWINSZ),
+		uintptr(unsafe.Pointer(&struct{ h, w, x, y uint16 }{uint16(h), uint16(w), 0, 0})))
+}