Re-pin Convex to 5e59870 (post upstream fix merge)

Upstream PR get-convex/convex-backend-skill#1 merged 2026-05-23. The
agents-field array-shape fix now applies; claude plugin validate passes
on both the full plugin (with marketplace.json) and the isolated
plugin.json — including the external-validator gate this PR previously
failed on.

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

.claude-plugin/marketplace.json

@@ -753,7 +753,7 @@
       "source": {
         "source": "url",
         "url": "https://github.com/get-convex/convex-backend-skill.git",
-        "sha": "59663a59be20c05911157116464f1be603ab70cc"
+        "sha": "5e59870cda2a5892e18a7164d1a46fcf57b70bea"
       },
       "homepage": "https://github.com/get-convex/convex-backend-skill",
       "keywords": [