The best PM specs in 2026 are 40-line markdown files in a git repo. Matt MacInnis, CPO at Rippling, said it: "No more planning decks, only markdown pushed to a git repo." Here's what that actually looks like. A PLANNING.md file with four sections: Problem: 2 sentences. Data-backed. Hypothesis: What you believe will happen and why. Success metrics: Specific thresholds. Not vibes. "Notification mute rate drops ≥15% vs control." Rollout: % exposure, duration, kill criteria. That replaced a…