ci(release): link agentex-client and agentex-sdk versions

The agentex-sdk wheel ships src/agentex/lib/**, but release-please
attributes those changes to agentex-client (path `.`), not agentex-sdk
(path adk/), so lib-only release windows never bump the SDK. Its rebuilt
wheel then collides with the immutable PyPI artifact at the unchanged
version (e.g. #426 / agentex-client-v0.16.1, where agentex-sdk stayed
0.15.0).

Add the linked-versions plugin so both components share one version and
bump together — matching the existing lockstep / floor-only-pin design.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: max-parke-scale

release-please-config.json

@@ -10,6 +10,16 @@
       "component": "agentex-sdk"
     }
   },
+  "plugins": [
+    {
+      "type": "linked-versions",
+      "groupName": "agentex",
+      "components": [
+        "agentex-client",
+        "agentex-sdk"
+      ]
+    }
+  ],
   "$schema": "https://raw.githubusercontent.com/stainless-api/release-please/main/schemas/config.json",
   "include-v-in-tag": true,
   "include-component-in-tag": true,