fix: refactor: drop over-engineered security modules; keep minimal safe quoting (fixes #1167)

[krystophny]

Summary

• Simplified gcov executor to avoid security wrapper modules and use minimal safe quoting + direct command execution.

Scope

• Touched src/gcov/gcov_executor.f90 only; no behavior changes to CLI or reporters.

Verification

• Ran fpm test locally (120s timeout). All listed tests pass:
  • markdown minimal, cli flags minimal, threshold minimal, gcov processing smoke, format/flags rejection.

Rationale

• Reduces over-engineered security layer surface area per refactor: drop over-engineered security modules; keep minimal safe quoting #1167 while retaining minimal path quoting where needed.