chore: add shared skills catalog (19 skills), installers, manifest, validator

skills/test-driven-development/SKILL.md

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+---
+name: test-driven-development
+description: Use when building new features, refactoring existing modules, or fixing bugs using Test-Driven Development (TDD: Red-Green-Refactor cycle).
+---
+
+# Test-Driven Development (TDD)
+
+## Purpose
+Build robust, well-designed, and thoroughly tested software by writing automated tests *before* writing production code.
+
+## The Red-Green-Refactor Cycle
+
+### 1. 🔴 RED — Write a Failing Test
+- Define the desired behavior from the caller's perspective.
+- Write a focused test asserting that behavior.
+- **Run the test** and verify that it fails for the *expected* reason (e.g. missing method or assertion mismatch, not a syntax/import error).
+
+### 2. 🟢 GREEN — Write the Minimal Implementation
+- Write just enough code to make the failing test pass.
+- Resist the temptation to implement speculative future features or premature optimizations.
+- **Run the test suite** and verify that all tests pass.
+
+### 3. 🔵 REFACTOR — Clean Up & Optimize
+- Eliminate duplication and code smells.
+- Improve naming, types, and module structure.
+- Ensure all invariants hold while keeping the test suite green at all times.
+
+## Best Testing Practices
+- **Test Behavior, Not Implementation Details:** Avoid asserting private state or internal methods. Test through public interfaces.
+- **Fast and Deterministic:** Unit tests should run in milliseconds without relying on real network calls or unstable timing delays.
+- **AAA Pattern:** Arrange (setup data), Act (call function/method), Assert (check outcome and side-effects).