Document actual test mocking convention (monkeypatch + unittest.mock)

[gbeane]

Summary

CLAUDE.md's Testing section lists pytest-mock as a tool and shows a mocker: MockerFixture example, but pytest-mock is not declared in any pyproject.toml and is not installed — the mocker fixture raises "fixture 'mocker' not found". This aligns the documentation with the actual, already-consistent convention.

Why docs, not the dependency

The codebase already mocks consistently without pytest-mock:

• 0 of 105 test files use mocker
• 29 use unittest.mock, 13 use monkeypatch

unittest.mock + the built-in monkeypatch fixture are stdlib / pytest built-ins and already the de-facto standard here. Adopting pytest-mock would mean adding the dep to all 5 workspace test groups and either a mixed convention or a 42-file migration for marginal ergonomic gain. The actual problem was the doc, not a missing tool.

Changes

• Remove pytest-mock from the Framework & Tools list; add an explicit "Mocking" bullet pointing to monkeypatch + unittest.mock and noting mocker is not available.
• Rewrite the mocker: MockerFixture example to use unittest.mock.Mock, plus a short monkeypatch.setattr(...) example for patching module-level symbols.

Docs-only; no code or tests affected.

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