docs(guides): add TDD workflow + restructure adding-a-feature for TDD order

New: docs/guides/tdd-workflow.md — red-green-refactor cycle, AAA,
mocking decision tree, coverage targets, factory + contract usage.
Restructured: adding-a-feature.md interleaves tests with implementation;
TDD order is required, not optional. testing-strategy.md cross-links
the new guide. AGENTS.md and CLAUDE.md surface both.

Spec: §7

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -11,6 +11,17 @@ pnpm turbo boundaries # Validate workspace dependency graph
 docker compose up -d  # Start PostgreSQL
 ```
 
+## TDD
+
+```bash
+pnpm test --watch --filter @repo/<feature>   # watch one feature
+pnpm test -- --coverage                      # full run with coverage
+pnpm test:stories                            # Storybook smoke tests
+pnpm test:e2e                                # Playwright e2e
+```
+
+See `docs/guides/tdd-workflow.md` for the full cycle.
+
 ## Project Overview
 
 Turborepo + pnpm monorepo organized by vertical features. Each feature (`auth`, `blog`, `media`, `marketing-pages`, `navigation`) owns its Clean Architecture layers. Core packages (`core-shared`, `core-cms`, `core-api`, `core-trpc`, `core-ui`) provide foundation. Two tooling packages (`core-eslint`, `core-typescript`) provide shared configs. Workspace boundaries are enforced by ESLint (lint-time) and Turborepo (build-graph time). Supports Next.js and TanStack Start as frontend frameworks, Payload CMS for content management, and comprehensive agent-optimized documentation.