Add Hindsight - AI agent memory by Vectorize

[nicoloboschi]

Summary

• Adds the Hindsight paper to the General Reasoning & Planning & Tool Using section.
• Hindsight is a state-of-the-art long-term memory system for AI agents by Vectorize (MIT-licensed, fully open source). It achieves 91.4% accuracy on LongMemEval and up to 89.61% on LoCoMo.
• Paper: "Hindsight is 20/20: Building Agent Memory that Retains, Recalls, and Reflects" (Dec 2025), a collaboration between Vectorize, Virginia Tech, and The Washington Post.