mbproxy/docs: retire superseded design/plan docs and dissolve DL260/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phase tree were point-in-time planning artefacts now superseded by the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/Proxy/BcdPduPipelineTests.cs

@@ -404,7 +404,7 @@ public sealed class BcdPduPipelineTests
 
     /// <summary>
     /// DL205/DL260 caps FC03/FC04 reads at qty=128 (above Modbus spec's 125; documented
-    /// in DL260/dl205.md). The proxy must NOT truncate the qty field — a request with
+    /// in docs/Reference/dl205.md). The proxy must NOT truncate the qty field — a request with
     /// qty &gt; 128 at non-BCD addresses must pass through unchanged so the PLC's own
     /// validator returns exception 03 to the client. This is the transparent-pass-through
     /// contract for FCs and addresses the rewriter doesn't own.