mbproxy/docs: retire superseded design/plan docs and dissolve DL260/

The standalone design.md, kpi.md, operations.md, and the docs/plan/
phase tree were point-in-time planning artefacts now superseded by the
topic-organized docs/ tree (Architecture/, Features/, Operations/,
Reference/, Testing/). The DL260/ folder mixed a device-reference doc, a
test fixture, a sample test, and a screenshot; its contents now live in
their natural homes (dl205.md + mbtcp_settings.JPG under docs/Reference/,
dl205.json next to its launcher in tests/sim/, sample test dropped).

All cross-references in the surviving docs, README, CLAUDE.md, the config
template, and source comments are repointed to the new locations.

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

mbproxy/tests/Mbproxy.Tests/Sim/SimulatorSmokeTests.cs

@@ -6,7 +6,7 @@ namespace Mbproxy.Tests.Sim;
 
 /// <summary>
 /// End-to-end smoke tests that verify the pymodbus DL205 simulator is reachable and
-/// serves the expected seeded register values from <c>DL260/dl205.json</c>.
+/// serves the expected seeded register values from <c>tests/sim/dl205.json</c>.
 /// </summary>
 /// <remarks>
 /// All three tests call <see cref="Assert.Skip"/> when