fix(opencode): disconnect dynamically added mcp servers

[rekram1-node]

Summary

• keep dynamically added MCP server configs in instance state
• let MCP status/connect/disconnect resolve runtime-added servers
• add HTTP API regression coverage and an SDK proof script for add/disconnect

Tests

• bun run prove:mcp-disconnect (packages/sdk/js)
• bun test test/server/httpapi-mcp.test.ts --timeout 30000 (packages/opencode)
• bun typecheck (packages/opencode)
• bun typecheck (packages/sdk/js)