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>

.gitignore

@@ -37,3 +37,6 @@ src/ZB.MOM.WW.OtOpcUa.Server/config_cache.db
 # E2E sidecar config — NodeIds are specific to each dev's local seed (see scripts/e2e/README.md)
 scripts/e2e/e2e-config.json
 config_cache*.db
+
+# Client CLI/UI runtime scratch (last-connected endpoint cache)
+session.dat