chore: tidy repo root — session/phase docs to docs/history, loose tests to tests/scratch

Item 25 of the Estate Tech-Debt Recovery Plan. Pure file relocation
(git mv, R100 — no content change):

- 9 session/phase/dated-progress .md (FIXES_2026-01-23, PHASE3-*,
  SESSION-*, MODULE-SYSTEM-PROGRESS, TRAIT_SYSTEM_STATUS, REPLY-SUM-TYPES)
  -> docs/history/ (joins the existing history docs there).
- 6 loose root test_*.affine snippets -> tests/scratch/.

Canonical root docs (README/SECURITY/CONTRIBUTING/CODE_OF_CONDUCT/
LICENSING-GUIDE/KNOWN-ISSUES/PROOF-NEEDS and design/reference docs) left
in place. Verified no build/CI/script references the moved paths.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyperpolymath