feat: add docs/ folder (architecture, guides, ADRs)

docs/decisions/adr-001-monorepo-tool.md

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+# ADR-001: Turborepo + pnpm for Monorepo
+
+## Status: Accepted
+
+## Context
+
+Need a monorepo tool for a clean architecture template with multiple apps and shared packages.
+
+## Decision
+
+Turborepo + pnpm workspaces.
+
+## Rationale
+
+- Clean architecture already provides organizational structure — Nx's opinions would compete
+- Minimal config (turbo.json + pnpm-workspace.yaml) means agents understand the setup quickly
+- Excellent caching — shared packages build once and are reused
+- Battle-tested pairing, both reference repos use it
+- Works naturally with Vite for non-Next packages