fix(agent): revive DEAD agents on heartbeat (not just STALE)

Reproduction: pause a container long enough to cross both the stale
and dead thresholds, then unpause. The agent resumes sending heartbeats
but the server keeps it shown as DEAD. Only a full container restart
(which re-registers) fixes it.

Root cause: AgentRegistryService.heartbeat() only revived STALE → LIVE.
A DEAD agent's heartbeat updated lastHeartbeat but left state unchanged.
checkLifecycle() never downgrades DEAD either (no-op in that branch),
so the agent was permanently stuck in DEAD until a register() call.

Fix: extend the revival branch to also cover DEAD. Same process; a
heartbeat is proof of liveness regardless of the previous state.

Also: AgentLifecycleMonitor.mapTransitionEvent() now emits RECOVERED
for DEAD → LIVE, mirroring its behavior for STALE → LIVE, so the
lifecycle timeline captures the transition.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cameleer-server-app/src/main/java/com/cameleer/server/app/agent/AgentLifecycleMonitor.java

@@ -70,7 +70,7 @@ public class AgentLifecycleMonitor {
     private String mapTransitionEvent(AgentState from, AgentState to) {
         if (from == AgentState.LIVE && to == AgentState.STALE) return "WENT_STALE";
         if (from == AgentState.STALE && to == AgentState.DEAD) return "WENT_DEAD";
-        if (from == AgentState.STALE && to == AgentState.LIVE) return "RECOVERED";
+        if (to == AgentState.LIVE && (from == AgentState.STALE || from == AgentState.DEAD)) return "RECOVERED";
         return null;
     }
 }

cameleer-server-core/src/main/java/com/cameleer/server/core/agent/AgentRegistryService.java

@@ -72,7 +72,10 @@ public class AgentRegistryService {
     /**
      * Process a heartbeat from an agent.
      * Updates lastHeartbeat, routeIds (if provided), capabilities (if provided),
-     * and transitions STALE agents back to LIVE.
+     * and revives STALE or DEAD agents back to LIVE. A DEAD revival happens
+     * when an agent resumes (e.g. docker unpause) after a pause long enough
+     * to cross both the stale and dead thresholds — the process is the same,
+     * no re-registration is required.
      *
      * @return true if the agent is known, false otherwise
      */
@@ -86,9 +89,9 @@ public class AgentRegistryService {
             if (capabilities != null && !capabilities.isEmpty()) {
                 result = result.withCapabilities(Map.copyOf(capabilities));
             }
-            if (existing.state() == AgentState.STALE) {
+            if (existing.state() == AgentState.STALE || existing.state() == AgentState.DEAD) {
                 result = result.withState(AgentState.LIVE).withStaleTransitionTime(null);
-                log.info("Agent {} revived from STALE to LIVE via heartbeat", id);
+                log.info("Agent {} revived from {} to LIVE via heartbeat", id, existing.state());
             }
             return result;
         });

cameleer-server-core/src/test/java/com/cameleer/server/core/agent/AgentRegistryServiceTest.java

@@ -116,6 +116,23 @@ class AgentRegistryServiceTest {
             assertThat(registry.findById("agent-1").state()).isEqualTo(AgentState.LIVE);
             assertThat(registry.findById("agent-1").staleTransitionTime()).isNull();
         }
+
+        @Test
+        void heartbeatDeadAgent_transitionsToLive() {
+            // A paused docker container can exceed both the stale and dead thresholds
+            // before resuming. When heartbeats arrive again the registry must revive
+            // the agent — otherwise the UI shows DEAD forever despite an alive agent.
+            registry.register("agent-1", "agent-1", "group", "default", "1.0.0", List.of(), Map.of());
+            registry.transitionState("agent-1", AgentState.STALE);
+            registry.transitionState("agent-1", AgentState.DEAD);
+
+            assertThat(registry.findById("agent-1").state()).isEqualTo(AgentState.DEAD);
+
+            registry.heartbeat("agent-1");
+
+            assertThat(registry.findById("agent-1").state()).isEqualTo(AgentState.LIVE);
+            assertThat(registry.findById("agent-1").staleTransitionTime()).isNull();
+        }
     }
 
     @Nested