[P2] Add Reproducible Rule Validation Harness

[barvhaim]

Roadmap Item 13: Add Reproducible Rule Validation Harness

Priority: P2
Area: Detection engineering
References: src/threatxtension/config/custom_semgrep_rules.yaml,
scripts/analyze_extension_patterns.py

Custom Semgrep rules are central to the project. They need a repeatable way to validate
true positives, false positives, and expected MITRE/CWE metadata.

Scope:

• Add rule fixtures with positive and negative JavaScript examples.
• Add tests that run Semgrep against fixture files.
• Validate rule metadata, severity, CWE, MITRE, and category fields.
• Document how to add a new rule safely.

Acceptance criteria:

• CI fails when custom rule syntax breaks or a fixture no longer triggers.
• Each high-value rule has at least one positive and one negative fixture.
• Rule authoring docs exist under docs/.

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Created from docs/roadmap.md.