feat(FN-1049): add per-agent heartbeat configuration via runtimeConfig

- Define AgentHeartbeatConfig interface in core types (heartbeatIntervalMs, heartbeatTimeoutMs, maxConcurrentRuns)
- Update HeartbeatMonitor to resolve per-agent config from AgentStore with validated min/max clamping
- Wire AgentStore into HeartbeatMonitor via InProcessRuntime initialization
- Add heartbeat settings section to dashboard AgentDetailView ConfigTab
- Add PATCH /api/agents/:id endpoint accepting runtimeConfig updates
- Add comprehensive tests for per-agent heartbeat config resolution and validation
- Document per-agent heartbeat configuration in AGENTS.md

packages/engine/src/runtimes/in-process-runtime.ts

@@ -213,6 +213,7 @@ export class InProcessRuntime
 
         this.heartbeatMonitor = new HeartbeatMonitor({
           store: this.agentStore,
+          agentStore: this.agentStore, // enables per-agent config resolution
           onMissed: (agentId) => {
             runtimeLog.warn(`Agent ${agentId} missed heartbeat`);
           },