feat: 30 - test mocking patterns

[miroslavpojer]

Adds the test-mocking-patterns skill — a focused reference for test double selection and implementation across Python, TypeScript/JavaScript, and Scala.

What it covers

• Double classification — when to use stub, mock, spy, fake, or dummy based on the nature of the dependency interaction (return value vs side effect vs unused)
• Python — pytest-mock / unittest.mock: patch targets, scope and cleanup, responses / pytest-httpx for HTTP stubbing, freezegun for time control, monkeypatch / patch.dict for env vars
• TypeScript / Jest — module mocking, hoisting behaviour, clearAllMocks / resetAllMocks / restoreAllMocks, spy vs mock distinction
• Scala / mockito-scala — stub, mock, spy, fake patterns with reset guidance
• Diagnostics — wrong patch target, mock state bleed, over-specified assertions, private-method testing antipattern, stub returning wrong type
• Don't mock what you don't own — guidance on wrapping third-party types in a thin protocol before mocking

What's included

Path	Description
skills/test-mocking-patterns/SKILL.md	Skill body
skills/test-mocking-patterns/evals/evals.json	Eval cases (happy path, regression, negative)
skills/test-mocking-patterns/evals/trigger-eval.json	Trigger boundary evals
skills/test-mocking-patterns/evals/fixture-map.md	Maps eval cases to source fixtures
skills/test-mocking-patterns/evals/files/	Source fixtures used by evals
docs/test-mocking-patterns.md	User-facing doc page

Closes #30