refactor: drop over-engineered security modules; keep minimal safe quoting

[krystophny]

Category: tech-debt
Priority: P2

PROBLEM

• Security modules and wrappers massively increase surface area without clear threat model; many tests are synthetic and don't reflect actual risk for CI coverage reporting.

EVIDENCE

• Files: src/security/, utils/command/ (secure wrappers), pervasive "SECURITY FIX" comments.
• gcov execution already runs via a simple command; extra layers add noise and maintenance.

SOLUTION

• Remove dedicated security modules; keep minimal validation/escaping where needed (e.g., quoting paths passed to gcov).
• Simplify gcov_executor to avoid bespoke secure wrappers; rely on careful argument quoting.

VERIFICATION

• Build succeeds; gcov execution works on typical projects.
• Keep one test verifying that filenames with spaces/pipes are handled safely.