Add BlackRock Advisor Center plugin (#6094)

Add the BlackRock Advisor Center skills plugin (blackrock/advisor-center-agent-skills, Apache-2.0). Skills-only: six SKILL.md workflows plus rendering/design-token references, plugin.json, and an .mcp.json pointing at the already-listed production Advisor Center MCP (https://ac360-mcp.blackrock.com/mcp, type http). No hooks, scripts or binaries. Pinned to main HEAD 66b3e980, whose tree is identical to the release commit the partner named (ad9a102b, not on main).

.claude-plugin/marketplace.json

@@ -589,6 +589,20 @@
       },
       "homepage": "https://boltz.bio"
     },
+    {
+      "name": "blackrock-advisor-center-plugin",
+      "description": "BlackRock Advisor Center 360° skills for financial advisors: portfolio review, opportunity and fund-health checks, guided benchmark selection, guided portfolio building, wealth projections, and routing to Advisor Center web workflows. Requires the BlackRock Advisor Center connector and an Advisor Center account.",
+      "author": {
+        "name": "BlackRock Advisor Center"
+      },
+      "category": "productivity",
+      "source": {
+        "source": "url",
+        "url": "https://github.com/blackrock/advisor-center-agent-skills.git",
+        "sha": "66b3e98002588a599c5c86fc10eea34b1dd93552"
+      },
+      "homepage": "https://github.com/blackrock/advisor-center-agent-skills"
+    },
     {
       "name": "box",
       "description": "Work with your Box content directly from Claude Code — search files, organize folders, collaborate with your team, and use Box AI to answer questions, summarize documents, and extract data without leaving your workflow.",