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>

CLAUDE.md

@@ -16,8 +16,7 @@ in this repo is .NET 10. PR 7.2 retired the legacy in-process
 `Galaxy.Host` / `Galaxy.Proxy` / `Galaxy.Shared` projects + the
 `OtOpcUaGalaxyHost` Windows service.
 
-See `lmx_mxgw.md` for the migration design and
-`docs/v2/Galaxy.Performance.md` for the runtime perf surface
+See `docs/v2/Galaxy.Performance.md` for the runtime perf surface
 (tracing, metrics, soak harness).
 
 ## Architecture Overview