docs(adr): ADR-011 TDD foundation; update AGENTS.md per-feature

Captures the decision to add @repo/core-testing, factories, contract
suites, vitest safety defaults, coverage thresholds, Storybook
test-runner, and CI as one cohesive TDD foundation. Per-feature
AGENTS.md gains a Tests section pointing to factories, contract suite,
and the canonical test commands.

Spec: §7.4, §7.5

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

packages/auth/AGENTS.md

@@ -75,6 +75,20 @@ beforeEach(() => {
 
 Covered areas: sign-in/up/out use cases, user validation, DI container binding.
 
+## Tests
+
+- **Factories:** `src/__factories__/user.factory.ts`, `src/__factories__/session.factory.ts` — use `userFactory.build({ overrides })` to construct test data with stable defaults.
+- **Contract suite:** `src/__contracts__/users-repository.contract.ts` — runs against every repository implementation (mock + payload).
+- **Unit tests:** colocated as `*.test.ts` next to the source file.
+- **Feature integration:** `tests/sign-in-flow.feature.test.ts` — full slice through tRPC router → controller → use case → mock repo.
+
+```bash
+pnpm test --filter @repo/auth           # all tests for this feature
+pnpm test --filter @repo/auth -- --watch # watch mode
+```
+
+See `docs/guides/tdd-workflow.md` for the cycle.
+
 ## Structure (minimal feature)
 
 ```