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;
     }
 }