fix(web): update workspace dependency from opencode to agent-core

The web package still referenced the old "opencode" workspace name,
causing bun install to fail with "Workspace dependency not found".

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/web/package.json

@@ -34,7 +34,7 @@
     "toolbeam-docs-theme": "0.4.8"
   },
   "devDependencies": {
-    "opencode": "workspace:*",
+    "agent-core": "workspace:*",
     "@types/node": "catalog:",
     "typescript": "catalog:"
   }