fix: detect hung agent processes via defensive signal.json polling

Claude CLI occasionally hangs after writing signal.json but never exits.
Add an optional signal check to pollForCompletion: after a 60s grace
period, check signal.json every 30s. If a valid completion signal is
found while the process is still alive, SIGTERM it and proceed to
normal completion handling.

apps/server/agent/manager.ts

@@ -369,6 +369,7 @@ export class MultiProviderAgentManager implements AgentManager {
       agentId, pid,
       () => this.handleDetachedAgentCompletion(agentId),
       () => this.activeAgents.get(agentId)?.tailer,
+      this.createEarlyCompletionChecker(agentId),
     );
     activeEntry.cancelPoll = cancel;
 
@@ -406,6 +407,20 @@ export class MultiProviderAgentManager implements AgentManager {
     return this.toAgentInfo(agent);
   }
 
+  /**
+   * Create a callback that checks if an agent has a valid signal.json,
+   * used by pollForCompletion to detect hung processes.
+   */
+  private createEarlyCompletionChecker(agentId: string): () => Promise<boolean> {
+    return async () => {
+      const agent = await this.repository.findById(agentId);
+      if (!agent?.worktreeId) return false;
+      const agentWorkdir = this.processManager.getAgentWorkdir(agent.worktreeId);
+      const signal = await this.outputHandler.readSignalCompletion(agentWorkdir);
+      return signal !== null;
+    };
+  }
+
   /**
    * Handle completion of a detached agent.
    */
@@ -525,6 +540,7 @@ export class MultiProviderAgentManager implements AgentManager {
       agentId, pid,
       () => this.handleDetachedAgentCompletion(agentId),
       () => this.activeAgents.get(agentId)?.tailer,
+      this.createEarlyCompletionChecker(agentId),
     );
     commitActiveEntry.cancelPoll = commitCancel;
 
@@ -633,6 +649,7 @@ export class MultiProviderAgentManager implements AgentManager {
         agentId, pid,
         () => this.handleDetachedAgentCompletion(agentId),
         () => this.activeAgents.get(agentId)?.tailer,
+        this.createEarlyCompletionChecker(agentId),
       );
       activeEntry.cancelPoll = cancel;
 
@@ -704,6 +721,7 @@ export class MultiProviderAgentManager implements AgentManager {
       agentId, pid,
       () => this.handleDetachedAgentCompletion(agentId),
       () => this.activeAgents.get(agentId)?.tailer,
+      this.createEarlyCompletionChecker(agentId),
     );
     activeEntry.cancelPoll = cancel;
 
@@ -890,6 +908,7 @@ export class MultiProviderAgentManager implements AgentManager {
       agentId, pid,
       () => this.handleDetachedAgentCompletion(agentId),
       () => this.activeAgents.get(agentId)?.tailer,
+      this.createEarlyCompletionChecker(agentId),
     );
     resumeActiveEntry.cancelPoll = resumeCancel;
   }
@@ -1013,6 +1032,7 @@ export class MultiProviderAgentManager implements AgentManager {
           agentId, pid,
           () => this.handleDetachedAgentCompletion(agentId),
           () => this.activeAgents.get(agentId)?.tailer,
+          this.createEarlyCompletionChecker(agentId),
         );
         const active = this.activeAgents.get(agentId);
         if (active) active.cancelPoll = cancel;