chore: add no-source-scanning test rule to CLAUDE.md

[jdalton]

Summary

• Adds a "Test Style — Functional Over Source Scanning" rule to the Testing section of CLAUDE.md
• Prohibits writing tests that read source files and assert on string contents (.toContain('pattern'))
• Directs toward functional tests (behavior verification), integration tests (built binary), and unit tests (real function calls)

Test plan

• Verify the new rule appears in the Testing section of CLAUDE.md
• Confirm no other files were modified

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Note

Low Risk
Low risk; this PR only updates developer documentation with testing guidance and does not change runtime code or test execution.

Overview
Adds a new “Test Style — Functional Over Source Scanning” section to CLAUDE.md that explicitly forbids tests which read source files and assert on string patterns (e.g., .toContain('pattern')).

Updates the testing guidance to steer contributors toward behavior-based functional tests, integration tests for built binaries, and unit tests that call real functions.

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