fix(mcp): consolidate all MCP servers at marketplace root

Move all MCP server declarations from individual plugin .mcp.json files
to a single .mcp.json at the marketplace root. This fixes MCP loading
failures where only one plugin's MCP would load.

- Add .mcp.json at marketplace root with all 5 servers
- Remove plugin-level .mcp.json files (projman, pr-review, cmdb-assistant,
  data-platform, viz-platform, contract-validator)
- Update CLAUDE.md to reflect new architecture

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

plugins/cmdb-assistant/.mcp.json

@@ -1,8 +0,0 @@
-{
-  "mcpServers": {
-    "netbox": {
-      "command": "${CLAUDE_PLUGIN_ROOT}/mcp-servers/netbox/run.sh",
-      "args": []
-    }
-  }
-}