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/core/src/agent-store.ts

@@ -10,7 +10,7 @@
  */
 
 import { mkdir, readFile, writeFile, readdir, unlink } from "node:fs/promises";
-import { existsSync } from "node:fs";
+import { existsSync, readFileSync } from "node:fs";
 import { join } from "node:path";
 import { randomUUID } from "node:crypto";
 import { EventEmitter } from "node:events";
@@ -732,6 +732,21 @@ export class AgentStore extends EventEmitter {
     return JSON.parse(content) as AgentData;
   }
 
+  /**
+   * Synchronously read an agent from disk (for use in synchronous hot paths).
+   * Returns null if the agent file does not exist or cannot be parsed.
+   * @param agentId - The agent ID
+   */
+  getCachedAgent(agentId: string): Agent | null {
+    try {
+      const path = join(this.agentsDir, `${agentId}.json`);
+      const content = readFileSync(path, "utf-8");
+      return this.parseAgent(JSON.parse(content) as AgentData);
+    } catch {
+      return null;
+    }
+  }
+
   private parseAgent(data: AgentData): Agent {
     return {
       id: data.id,