fix(codex): isolate subagent app-server events

[AaronZ345]

Summary

• ignore Codex app-server lifecycle notifications that belong to subagent threads instead of the active cc-connect thread
• scope item, completion, and token-usage notifications to the active parent turn
• preserve compatibility with older app-server notifications that omit thread or turn IDs
• add regression coverage proving child turns cannot overwrite or complete the parent turn

Root cause

Codex app-server multiplexes parent and subagent events over the same connection. Its v2 protocol includes threadId on turn/started, turn/completed, and thread/status/changed, plus both threadId and turnId on item and token-usage notifications.

cc-connect parsed those fields but did not validate them. When a subagent started, its turn/started notification replaced the parent's currentTurn and cleared pending parent messages. When that child thread became idle or completed, cc-connect called completeTurn() and emitted an empty EventResult. Repeated child turns therefore produced repeated empty responses, while parent output could be truncated. Child token usage could also overwrite the parent turn's usage snapshot.

This change fixes the event-routing boundary rather than suppressing empty messages. Parent-thread follow-up turns, including results delivered back from subagents, continue through the normal unsolicited-event path.

Live evidence

In a GPT-5.6 Sol ultra run, the parent turn completed at 12:15:08. Codex then started subagent turns on distinct thread IDs. Child thread idle transitions at 12:15:15 and 12:15:23 aligned exactly with cc-connect logging unsolicited turn complete response_len=0. The Codex 0.144.1 generated protocol schema confirms these notifications carry the thread and turn identifiers needed for routing.

Validation

• go test ./agent/codex -count=1
• go test -race ./agent/codex -run 'TestAppServerSession_(IgnoresSubagentLifecycleNotifications|IgnoresSubagentTokenUsageNotifications)' -count=1
• go test -tags no_web ./cmd/cc-connect ./agent/codex -count=1
• go vet ./agent/codex
• git diff --check

[AaronZ345]

Hi @chenhg5, I’ve rebased this PR onto the latest main and all five CI checks are green. Could you please take another look and merge it when you have a chance? Thanks!