fix: avoid duplicate plugin MCP registration

[MukundaKatta]

Summary:

• remove the duplicate chrome-devtools MCP server declaration from .claude-plugin/plugin.json
• keep .mcp.json as the single source of truth for plugin MCP registration
• avoid Claude Code /doctor warnings when the marketplace plugin is installed

Testing:

• node -e "JSON.parse(require('fs').readFileSync('.claude-plugin/plugin.json','utf8'))"

Fixes #1854

[OrKoN]

We should delete .mcp.json instead and keep the configuration within individual plugins to support things like #1851