W18: context cleanup + CLI fixes + loader audit + CI matrix (W18.1-W18.4)

- W18.1 (王测+林深): Remove g_max_tokens dead API, UTF-8 bounds protection, deduplicate token counting, 0xC0/0xC1 handling, add 13 test blocks (36 checks)
- W18.2 (赵码+朱晴): Fix /context no-session error message, /status 3-state connection display
- W18.3 (曹武+徐磊): plugin_loader security audit — 9 dimensions, rating C, 1 HIGH + 2 MEDIUM findings
- W18.4 (马奔+胡桐): CI dual-platform matrix (Ubuntu clang-18 + Windows clang-cl), ccache, build timing baseline

Build 0 error, ctest 5/5 pass, metadata check clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CMakePresets.json

@@ -30,6 +30,18 @@
             },
             "toolchainFile": "generators\\conan_toolchain.cmake",
             "binaryDir": "E:\\Prj2026\\AIGen2026\\dstalk\\build\\build\\Release"
+        },
+        {
+            "name": "ci-release",
+            "displayName": "CI Release (cross-platform)",
+            "description": "Cross-platform CI build; compiler set via Conan CMakeToolchain",
+            "generator": "Ninja",
+            "cacheVariables": {
+                "CMAKE_POLICY_DEFAULT_CMP0091": "NEW",
+                "CMAKE_BUILD_TYPE": "Release"
+            },
+            "toolchainFile": "${sourceDir}/build/Release/conan_toolchain.cmake",
+            "binaryDir": "${sourceDir}/build/ci"
         }
     ],
     "buildPresets": [
@@ -37,6 +49,11 @@
             "name": "conan-release",
             "configurePreset": "conan-release",
             "jobs": 32
+        },
+        {
+            "name": "ci-release",
+            "configurePreset": "ci-release",
+            "jobs": 0
         }
     ],
     "testPresets": [
@@ -49,6 +66,13 @@
             "environment": {
                 "OPENSSL_MODULES": "C:\\Users\\Administrator\\.conan2\\p\\b\\opens4d81e45a1d5f5\\p\\lib\\ossl-modules"
             }
+        },
+        {
+            "name": "ci-release",
+            "configurePreset": "ci-release",
+            "execution": {
+                "jobs": 0
+            }
         }
     ]
 }