feat: v2026.3.20 — fix missing Responses API export

The responses endpoint added in v2026.3.18 was not exported from the
endpoints barrel, making it inaccessible to SDK consumers. Also removes
unused README-IMPROVED.md.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: georgeglarson