test: add initial vulnerable and benign skill fixtures

[kamalsrini]

Summary

• Adds initial true-positive and false-positive fixtures for five high-value skills
• Covers secure-code-review, dependency-scanning, secrets-management, api-security, and prompt-injection
• Adds one vulnerable and one benign case per covered skill

Fixture Matrix

Skill	Vulnerable	Benign
secure-code-review	sql-injection-vulnerable	parameterized-query-benign
dependency-scanning	vulnerable-npm-lock	benign-npm-lock
secrets-management	hardcoded-test-secret-vulnerable	example-env-benign
api-security	missing-admin-auth-vulnerable	admin-auth-benign
prompt-injection	retrieval-instructions-vulnerable	retrieved-content-data-benign

Linear

• UNI-10

Verification

• ruby scripts/test_skill_fixtures.rb
• ruby scripts/validate_index.rb
• ruby scripts/validate_skill_schema.rb
• secret-pattern scan over tests/fixtures
• git diff --check HEAD