Move field report to docs/history; refresh README repository layout

- docs/history/echo-improvements-prompt.md (ROADMAP-2.0 §5 box ticked)
- README repository layout brought to 1.5.1 reality: root docs
  (CHANGELOG/TODO-1.6/ROADMAP-2.0/docs/history), build.py + dist/
  (gitignored, secret-bearing), current eval harness contents, hooks/,
  and the full scripts/ roster (recall/reflect/triage/queue/
  concurrency/quality/output/doctor/hook modules) with the legacy
  commands/ dir annotated as the TODO-1.6 migration target. Also
  restores the eval-descriptor fix lost in the prior commit (edit was
  applied in memory but never written back).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ROADMAP-2.0.md

@@ -69,9 +69,8 @@ the README. Re-run it per release and refresh the README table.
 - [x] Retire `ROADMAP-1.0.md` (fully shipped; in git history) — **done 2026-07-03**.
 - [ ] Root README "Repository layout" section updated for the post-2.0 tree
       (no versioned zips, no codex tree, skills-only plugin).
-- [ ] Confirm `echo-improvements-prompt.md` (the 1.5.0 field report) either moves to a
-      `docs/history/` folder or is deleted — its content is preserved in
-      CHANGELOG 1.5.0 and the git history.
+- [x] `echo-improvements-prompt.md` (the 1.5.0 field report) moved to `docs/history/`
+      — **done 2026-07-03**.
 
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