docs: drop list-endpoint perf planning docs

Three planning notes from 3f34b78 were committed under docs/planning/
(wrong convention — should be docs/claude/planning/ per CLAUDE.md) and
have since rotted: PR-A was superseded by the denormalize approach in
#1301; the .only() recommendation was reverted per review; PR-B1 ships
in this PR. Two genuinely useful items have been harvested:

- New Relic baseline numbers for 9 list endpoints (post-NR-12.1.0)
  → memory entry reference_nr_baseline_2026_05_11.md
- 22:22 UTC PG connection-pool burst hypothesis
  → issue #1302
- SourceImageViewSet.retrieve 571-call N+1 audit
  → issue #1303

Co-Authored-By: Claude <noreply@anthropic.com>

Author: mihow