* fix(tests): remove unused test constants and helpers
Delete dead test code that caused 41 staticcheck U1000 violations:
- cli/test_constants.go (25 unused constants)
- cli/terminal_test_helpers.go (unused type, method, 7 variables)
- fileproc/test_constants.go (5 unused constants)
- fileproc/processor_test.go (2 unused helper functions)
* fix(security): replace custom secret detection with gitleaks
The hand-rolled check_secrets regex patterns produced false positives
on configKey test values, causing make security-full to fail.
Replace with gitleaks via go run for proper secret detection with
built-in rules and allowlist support for generated report files.
* chore(deps): update dependencies and fix install-tools
Update Go module dependencies to latest versions.
Fix checkmake install path and remove yamllint go install
(yamllint is a Python tool, not installable via go install).
* docs: add design document for gitleaks integration
* feat: update go to 1.25.6
