feat: metrics agent for FreeSWITCH and Asterisk nodes

- pulse-lets-go-agent: collects metrics from PBX nodes via ESL (FreeSWITCH) or AMI (Asterisk)
- AMI client (raw TCP) for Asterisk Manager Interface
- Collector interface: collect, connect, subscribe hangup events, close
- FreeSWITCH collector: show channels, max_sessions, idle_cpu
- Asterisk collector: CoreShowChannels, CoreSettings
- System collector: /proc/loadavg, /proc/stat (CPU idle)
- Failure tracker: sliding window for call_failure_rate
- NATS publisher: pulse.metrics.<node_id> every 5 seconds
- Graceful shutdown, retry logic (5 failures → status=down)
- Template configs for UC nodes (Asterisk) and ses-sip (FreeSWITCH)

contrib/agent-sessip.json

@@ -0,0 +1,15 @@
+{
+  "node_id": "ses-sip",
+  "type": "freeswitch",
+  "nats_url": "nats://balancer.lan:4222",
+  "interval_sec": 5,
+  "max_calls": 250,
+  "sip_gateway": "sip:ses-pbx.lan:5060",
+  "sip_gateway_auto": false,
+  "failure_window": 1000,
+  "esl": {
+    "host": "127.0.0.1",
+    "port": 8021,
+    "password": "ClueCon"
+  }
+}

contrib/agent-uc.json

@@ -0,0 +1,16 @@
+{
+  "node_id": "uc06",
+  "type": "asterisk",
+  "nats_url": "nats://balancer.lan:4222",
+  "interval_sec": 5,
+  "max_calls": 150,
+  "sip_gateway": "sip:uc-pbx.lan:5060",
+  "sip_gateway_auto": false,
+  "failure_window": 1000,
+  "ami": {
+    "host": "127.0.0.1",
+    "port": 6154,
+    "username": "admin",
+    "password": "changeme"
+  }
+}