fix: treat signal-killed agents as success when output was delivered

Two bugs fixed:

1. agent-runner: process close handler only checked `code !== 0`,
   but signal kills (SIGTERM/SIGKILL from post-close cleanup) set
   code=null which was misclassified as error even after successful
   output delivery. Now checks `code === null && signal` and resolves
   as success when hadStreamingOutput is true.

2. message-agent-executor: wrappedOnOutput persisted newSessionId
   before checking for poisoned session, allowing a stale session to
   be re-saved after clearSession. Reordered to check poison first
   and guard persist with !resetSessionRequested.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/message-agent-executor.ts

@@ -140,9 +140,6 @@ export async function runAgentForGroup(
 
     const wrappedOnOutput = onOutput
       ? async (output: AgentOutput) => {
-          if (persistSessionIds && output.newSessionId) {
-            deps.persistSession(group.folder, output.newSessionId);
-          }
           if (
             persistSessionIds &&
             isClaudeCodeAgent &&
@@ -150,6 +147,13 @@ export async function runAgentForGroup(
           ) {
             resetSessionRequested = true;
           }
+          if (
+            persistSessionIds &&
+            output.newSessionId &&
+            !resetSessionRequested
+          ) {
+            deps.persistSession(group.folder, output.newSessionId);
+          }
           const evaluation = evaluateStreamedOutput(output, streamedState, {
             agentType: isClaudeCodeAgent ? 'claude-code' : 'codex',
             provider,