feat: add raven stub, state tracking, sam-research + searxng monitoring

- RAVEN_URL env var: sends down/recovery alerts when Raven is live (silent no-op until then)
- service-states.json persists prev state for change detection
- config.yaml: adds sam-research + searxng services, adds sam/citadel/varys agents

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config.yaml

@@ -8,6 +8,10 @@ services:
     url: http://hodor-gateway:8200/health
   - name: citadel-mcp
     url: http://citadel-mcp:8300/sse
+  - name: sam-research
+    url: http://sam-research:8500/health
+  - name: searxng
+    url: http://searxng:8080
 
   # Core services
   - name: open-webui
@@ -34,3 +38,6 @@ services:
 # One-shot agents to watchdog (reads /opt/agent-os/logs/<name>/last-run.json).
 agents:
   - name: bran-changelog
+  - name: sam-research
+  - name: citadel-mcp
+  - name: varys-monitor