fix(packaging): resolve runtime deps from plugin package

[vicary]

Summary

• resolve cosmiconfig and @modelcontextprotocol/sdk relative to the plugin package instead of process.cwd()
• add MCP SDK to generated npm package dependencies and cover the generated metadata in packaging tests
• exercise package-name loading from a bare cwd in Node/OpenCode packaging regression coverage

Test Plan

• deno task build
• deno task check
• deno task lint
• deno task fmt
• deno test -A packaging.test.ts
• deno test -A