chore: add kiro spec-driven tooling and steering docs

Add the Kiro-style agentic SDLC tooling used to drive this project: the
.opencode/skills/kiro-* skill set (discovery, spec init/requirements/design/
tasks, validation, implementation, review, steering), the .kiro/settings
templates, and AGENTS.md describing the workflow. Update the spec docs
(assessment/design/requirements/tasks) to current state.

Scope tests/test_opencode_skills.py to the project's own openultrasast-*
skills so the third-party kiro-* tooling (which uses a different frontmatter
convention) no longer breaks the frontmatter check.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

.kiro/settings/templates/steering-custom/testing.md

@@ -0,0 +1,47 @@
+# Testing Standards
+
+[Purpose: guide what to test, where tests live, and how to structure them]
+
+## Philosophy
+- Test behavior, not implementation
+- Prefer fast, reliable tests; minimize brittle mocks
+- Cover critical paths deeply; breadth over 100% pursuit
+
+## Organization
+Options:
+- Co-located: `component.tsx` + `component.test.tsx`
+- Separate: `/src/...` and `/tests/...`
+Pick one as default; allow exceptions with rationale.
+
+Naming:
+- Files: `*.test.*` or `*.spec.*`
+- Suites: what is under test; Cases: expected behavior
+
+## Test Types
+- Unit: single unit, mocked dependencies, very fast
+- Integration: multiple units together, mock externals only
+- E2E: full flows, minimal mocks, only for critical journeys
+
+## Structure (AAA)
+```typescript
+it('does X when Y', () => {
+  // Arrange
+  const input = setup();
+  // Act
+  const result = act(input);
+  // Assert
+  expect(result).toEqual(expected);
+});
+```
+
+## Mocking & Data
+- Mock externals (API/DB); never mock the system under test
+- Use factories/fixtures; reset state between tests
+- Keep test data minimal and intention-revealing
+
+## Coverage
+- Target: [% overall]; higher for critical domains
+- Enforce thresholds in CI; exceptions require review rationale
+
+---
+_Focus on patterns and decisions. Tool-specific config lives elsewhere._