Document external probes and count adoption in telemetry

Add availability_probe_targets and availability_probe_agents to the
telemetry ping - counts only, no agent names or addresses - with the
disclosure table updated in both privacy doc copies. Document the
feature in the availability-checks configuration guide and the
unified agent guide, including the ICMP capability caveat for
containerized probes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/UNIFIED_AGENT.md

@@ -84,9 +84,34 @@ curl -fsSL http://<pulse-ip>:7655/install.sh | \
 - **Host Metrics**: CPU, memory, disk, network I/O, temperatures
 - **Docker Monitoring**: Container metrics, health checks, Swarm support (when enabled)
 - **Kubernetes Monitoring**: Cluster, node, pod, and deployment health (when enabled)
+- **External Probes** (Pro): runs availability checks assigned to this agent from the Pulse server and reports the results back — see below
 - **Auto-Update**: Automatically updates when a new version is released
 - **Multi-Platform**: Linux, macOS, Windows support
 
+## External Probes (Pro)
+
+With the Pro `external_probe` entitlement, availability checks configured in
+Pulse can be assigned to run from a specific agent instead of the Pulse
+server (Settings -> Monitoring -> Availability checks -> "Run from"). This is
+how you monitor a site from the outside: deploy the agent on a machine
+elsewhere — a cloud VM, a Docker host at another location — and assign checks
+to it. Alerting still happens on the Pulse server through your normal alert
+routes.
+
+There is nothing to configure on the agent itself. Assignments arrive through
+the agent's signed remote configuration, the agent runs each check on its
+configured interval, and results are delivered with its regular reports.
+Results survive temporary connectivity loss to the Pulse server in a bounded
+in-memory queue; if the agent cannot deliver for several check intervals the
+check shows as indeterminate in Pulse until reports resume. The module
+appears as `availability` in the agent's module status when at least one
+check is assigned.
+
+Note for ICMP (ping) checks: the probe uses the system `ping` binary. In
+containers or hardened service units without `CAP_NET_RAW`, ICMP checks fail;
+prefer TCP or HTTP checks there, or grant the capability. See "ICMP probe
+privileges" in docs/CONFIGURATION.md.
+
 ## Configuration
 
 | Flag | Env Var | Description | Default |