He weighs campaign for secretary of stateCameron F. Kerry, the younger brother and a close political adviser to US Senator John F. Kerry, said yesterday he is laying the groundwork to run for the 2006 Democratic nomination for secretary of state. Kerry, a Boston lawyer who has operated in his sibling's shadow for more than three decades, said he met with Secretary of State William F. Galvin yesterday to inform him of his decision to run for the office, but assured him it was dependent on Galvin's vacating the post to run for the Democratic nomination for governor.