✨ Add testing skill with TDD workflow enforcement

Minimal skill referencing mem:testing for full guidance, covering
TDD Red-Green-Refactor cycle, Prove-It pattern for bug fixes,
and key testing principles.

AI-assisted-by: mimo-v2.5-pro

.opencode/skills/testing/SKILL.md

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+---
+name: testing
+description: Enforce TDD workflow and testing best practices for Penpot. Use when implementing features, fixing bugs, or modifying behavior. Reads testing memory for full guidance.
+---
+
+# Testing Skill
+
+Enforces test-driven development and Penpot testing conventions.
+
+## When to Use
+
+- Implementing new logic or behavior
+- Fixing any bug (reproduction test required)
+- Modifying existing functionality
+- Adding edge case handling
+
+**Skip:** Pure configuration changes, documentation updates, or static content with no behavioral impact.
+
+## Workflow
+
+Follow TDD (Red → Green → Refactor) whenever practical:
+
+1. **RED** — Write a failing test first
+2. **GREEN** — Write minimal code to pass
+3. **REFACTOR** — Clean up while tests stay green
+
+For bug fixes, use the Prove-It Pattern: write a test that reproduces the bug, confirm it fails, implement the fix, confirm it passes.
+
+## Required Reading
+
+Before writing any test, read:
+
+1. `.serena/memories/testing.md` — cross-cutting testing principles, TDD workflow, anti-patterns, execution discipline
+2. Module-specific testing memory for the affected module:
+   - `mem:common/testing` — CLJC unit tests
+   - `mem:frontend/testing` — CLJS unit tests, Playwright E2E
+   - `mem:backend/core` — JVM clojure.test conventions
+
+## Key Rules
+
+- Every behavior change needs a test
+- Test state, not interactions
+- DAMP over DRY — tests are specifications; duplication is OK if each test is self-contained and readable
+- Prefer Real > Fake > Stub > Mock
+- Arrange-Act-Assert structure
+- One assertion per concept
+- Never pipe test output to filters — redirect to file first
+- Register new test files in the module's runner/entrypoint
+
+## Verification
+
+After completing implementation:
+
+- [ ] Every new behavior has a test
+- [ ] All tests pass for touched modules
+- [ ] Bug fixes include a reproduction test
+- [ ] Lint/formatter passes