fix: support current Codex subagent activity events

[yunhungo]

Summary

• recognize current Codex sub_agent_activity events with kind: started and agent_thread_id
• preserve the legacy collab_agent_spawn_end schema and deduplicate child thread IDs
• add regression coverage proving a child rollout and its exec_command action are nested under the parent trace

Root cause

Codex now emits successful subagent starts as sub_agent_activity, while the resolver only recognized collab_agent_spawn_end. The child rollout existed on disk but its thread ID was never collected, so it was not resolved into the parent trace.

Validation

• pnpm exec vitest run plugins/tracing/test/parse.test.ts plugins/tracing/test/trace.test.ts (20 passed)
• NODE_OPTIONS=--no-warnings pnpm test (37 passed)
• NODE_OPTIONS=--no-warnings pnpm lint

Fixes #41

[CLAassistant]

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