chore: remove stale dated session notes (DONE work pre-2026-04-21)

Removes 4 session-close audits from 2026-04-17 and the 2026-03-28
session summary. All described completed work and are superseded by
CHANGELOG/README and audit-trail records.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath