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/media/AGENTS.md

@@ -69,6 +69,18 @@ beforeEach(() => {
 - **Alias** — `@/` resolves to `src/`
 - **Run** — `pnpm test --filter @repo/media`
 
+## Tests
+
+- **Factories:** `src/__factories__/media.factory.ts` — use `mediaFactory.build({ overrides })` to construct test data with stable defaults.
+- **Unit tests:** colocated as `*.test.ts` next to the source file.
+
+```bash
+pnpm test --filter @repo/media           # all tests for this feature
+pnpm test --filter @repo/media -- --watch # watch mode
+```
+
+See `docs/guides/tdd-workflow.md` for the cycle.
+
 ## Structure (minimal)
 
 ```