(This Sept. 13 story corrects name of group to “Investor Agenda,” not “Investor Alliance,” in last paragraph) By Ross Kerber and Simon Jessop BOSTON/LONDON (Reuters) – Investors managing $39 trillion have called on governments to raise their climate ambition, including setting plans to phase out fossil fuel use and forcing companies to set out science-based transition plans. The move by some – but not all – top fund firms comes ahead of the next round of global climate talks in Egypt in November. This year’s letter is the most ambitious appeal to officials yet, backers of the effort said, with…