Report agent liveness

Refreshes the calling agent's status. Workers may send X-Runtime-Instance-ID, the per-boot identifier of the calling process. It is ignored unless MULTI_RUNTIME_ENABLED is set. With multi-runtime mode on, the header must identify a live runtime of this agent; an absent, unknown, offline, or foreign identifier makes the call a no-op instead of an error, so workers predating the flag keep running.