Move the ICMP, TCP, HTTP and UDP probe core from the monitoring
availability poller into internal/availabilityprobe so the host agent
can execute the same checks without importing the monitoring package.
Monitoring keeps type aliases and thin wrappers, so callers and the
canonical guardrail contracts are unchanged. Pure refactor for the
upcoming external probe feature.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>