fix(agent): use session key agentId for transcript path

Cross-agent subagent spawns wrote transcripts to the spawner's agent
directory instead of the target agent's directory. For example, when
main spawned a codex subagent with session key agent:codex:subagent:...,
the transcript went to agents/main/sessions/ instead of agents/codex/sessions/.

Pass sessionAgentId to resolveSessionFilePath so transcripts are written
to the correct agent's session directory.

src/commands/agent.ts

@@ -416,7 +416,9 @@ export async function agentCommand(
       catalog: catalogForThinking,
     });
   }
-  const sessionFile = resolveSessionFilePath(sessionId, sessionEntry);
+  const sessionFile = resolveSessionFilePath(sessionId, sessionEntry, {
+    agentId: sessionAgentId,
+  });
 
   const startedAt = Date.now();
   let lifecycleEnded = false;