Bret Taylor’s AI startup Sierra is raising a $950 million funding round led by Tiger Global and GV, the company announced Monday , pushing its post-money valuation above $15 billion. The raise gives Sierra more than $1 billion to work with — capital the company says it will use to become the “global standard” for AI-powered customer experiences. Like a lot of AI companies, Sierra has, smartly, been very proactive in touting its own growth in a crowded market. The company says it started with just four design partners a couple of years ago. Today it claims to have more than 40% of the Fortune 50 as customers, and says the agents running on its platform are handling billions of interactions, from refinancing mortgages to processing insurance claims, managing returns, and powering nonprofit fundraising campaigns.…