fix(cli): serialize fanout lifecycle events

[chubes4]

Summary

• serialize fanout lifecycle journal appends in callback order
• preserve monotonic active/completed progress snapshots under concurrent workers
• retain the existing end-to-end journal assertion that exposed the release-blocking race

Fixes #2439.

Verification

• npx tsx tests/agent-fanout-executor.test.ts repeated 25 times
• npm run build
• git diff --check

AI assistance

GPT-5.6-sol through OpenCode traced the failed v0.26.3 release preflight to concurrent lifecycle appends, implemented serialization at the journal boundary, and ran the listed verification. Chris Huber directed the release and reviewed the objective.