Clarify SEN publication scope and editorial transition

[yqtian-se]

Summary

• clarify that SEN does not accept unsolicited manuscripts
• distinguish commissioned columns and pre-arranged workshop publications from general submissions
• align the communications playbook with the revised SEN scope
• list Yongqiang Tian as incoming Editor-in-Chief from 2027

Validation

• git diff --check upstream/main...HEAD
• docker compose run --rm hugo hugo

[yqtian-se]

We got many emails about submitting technical papers to SEN, which is not expected at this moment. We may have more columns in the future, after we finalise the protocols.