/.well-known/mcp.json and /llms.txt name the Claude Code and OpenCode plugins, each with its one install line #8273

The forge ships thin plugins for Claude Code and OpenCode in plugins/, but an agent or a person who finds the forge through /.well-known/mcp.json or /llms.txt learns nothing about them. Being beside every harness means the harness can find them.

Done when:

• [ ] The MCP card (src/core/mcp_card.ts) carries a plugins list: for each plugin, its harness, where its source is in the repository, and its one install line (claude --plugin-dir …; OpenCode's plugin entry), built from one constant
• [ ] /llms.txt says the same in one short section, from the same constant, so the two cannot drift
• [ ] A test fetches both and proves each plugin is named with its install line, and that every named path exists in plugins/. It fails without the change

Files: src/core/mcp_card.ts, src/web/standards.ts, test/mcp-card-plugins.test.ts Risk: the card format is a draft (see mcp_card.ts). Add the field without renaming existing ones.