fix: move mcp_servers to metadata.json to prevent silent plugin.json rejection

Claude Code's strict schema validation silently rejects plugins with unknown
root-level fields in plugin.json (anthropics/claude-code#20409).
Moved mcp_servers to separate metadata.json files.
Updated install/uninstall/list scripts to read from new location.
Added cache clearing to switch-profile.sh.

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

plugins/data-platform/.claude-plugin/metadata.json

@@ -0,0 +1 @@
+{"mcp_servers": ["data-platform"]}