feat: add explicit tracked subsession yielding

[jmfederico]

Summary

• add yield_to_subsessions as an explicit tracked-subsession join action
• list other working children in each completion notice so the parent can work or yield again
• keep working-child inspection non-terminating and withhold partial model-facing output
• place PI WEB guidance before a labeled marker and child output/transcripts last
• use concise tool guidance, with focused tests, synchronized docs, and a patch Changeset

Validation

• npm run verify
• npm run build
• git diff --check origin/main...HEAD

Operational note

Restart the long-lived pi-web-sessiond.service after deploying this change.