chore: drop root scratch + retired v2-mxgw plan docs

- Delete _p54.json / _p55.json (PR-body snapshots for the shipped S7
  + Mitsubishi research docs).
- Delete session.dat (38-byte CLI runtime cache, not produced by any
  current source code) and add it to .gitignore so it doesn't come
  back.
- Delete lmx_backend.md / lmx_mxgw.md / lmx_mxgw_impl.md. All three
  carried "✅ Completed 2026-04-30" historical-record banners — the
  v2-mxgw migration shipped + merged to master, so the design plans
  served their purpose. Drop the cross-refs from CLAUDE.md and
  docs/v1/README.md.

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

docs/v1/README.md

@@ -15,7 +15,6 @@ For current architecture see:
 - `docs/drivers/Galaxy.md` — current Galaxy driver doc
 - `docs/v2/Galaxy.ParityRig.md` — current testing setup
 - `docs/v2/Galaxy.Performance.md` — observability + perf
-- `lmx_mxgw.md` (in repo root) — design rationale for the migration
 
 | File | What it covered |
 |---|---|