The dominant 2026 second-brain architecture is vault-as-foundation (plaintext markdown knowledge base) with Claude Code as engine—repo-as-database means knowledge organization directly impacts reasoning quality. Maintenance kills PKM systems, so Claude maintains via self-feeding read/write loops while humans write pure, honest content. At scale, hierarchical model routing (80% of tasks are janitorial and don't need frontier intelligence) cuts costs 10x. The scratchpad/napkin pattern compounds agent memory across sessions; agents logging mistakes improve over time. Four independent team-knowledge implementations (DoorDash, Pendo, Google, solo builders) converged on the same three-layer architecture: call summaries, decision logs, and analytics queries returning full reasoning in 15 seconds, addressing institutional knowledge loss (47% of companies' top offboarding challenge, 6–7 months to onboard, 8+ productive hours lost weekly to context questions). Obsidian plus Claude Code skills create persistent knowledge systems where /skills map to file-based architecture; the three-folder schema (raw/, wiki/, outputs/) plus CLAUDE.md outperforms fancy tooling. Tolaria (git-based, plain markdown with visual version history) exemplifies the vault pattern extended to human–agent collaboration without vendor lock-in. The convergence signal is data compounding, not technology, being the moat.