chore: remove stale dated session summary

Remove SESSION-SUMMARY-ECHIDNABOT-2026-01-25.md describing DONE
cleanup work from 2026-01-25 (well before 2026-04-21 cutoff).
Architecture lessons and policy are already preserved in
.claude/CLAUDE.md and ADR-008/009.

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

Author: hyperpolymath