test: Add comprehensive test coverage for all hooks

## Overview
Extend the bats testing framework to provide comprehensive test coverage for all 26 hook scripts.

## Current State
- Testing framework established with bats
- Sample tests created for:
- `hooks/commits/gitlint.sh`
- `hooks/nix/nix-fmt.sh`
- Test helper utilities available
- CI integration configured

## Requirements

### Test Coverage Goals
- [ ] 100% of hooks have at least basic tests
- [ ] 80%+ code coverage across all hooks
- [ ] Edge cases and error conditions tested
- [ ] Multi-platform compatibility verified

### Hooks Needing Tests

#### Commits
- [ ] commitizen.sh
- [ ] commitlint.sh
- [x] gitlint.sh ✅

#### Nix
- [ ] attestation.nix
- [ ] nix-build-check.sh
- [ ] nix-build.sh
- [ ] nix-darwin-check.sh
- [ ] nix-flake-check.sh
- [x] nix-fmt.sh ✅
- [ ] nix-home-manager-check.sh
- [ ] nix-lint.sh
- [ ] publish-to-nixpkgs.sh

#### CI
- [ ] github.sh
- [ ] gitlint.sh

#### Configs
- [ ] yamlfmt.sh

#### Terraform
- [ ] pluralith.sh
- [ ] tfupdate.sh

#### Web
- [ ] css/csslint.sh
- [ ] js/eslint.sh
- [ ] js/fixmyjs.sh
- [ ] js/jshint.sh
- [ ] scss/scss-lint.sh

#### Other
- [ ] backup-nixos.sh
- [ ] update-vm.sh
- [ ] witness.sh
- [ ] pluralith.sh (root)

## Test Structure
Each test file should include:
1. Existence and executability check
2. Missing dependency handling
3. Success case validation
4. Failure case validation
5. Edge case handling (empty input, malformed data, etc.)

## Deliverables
- [ ] Complete test suite for all hooks
- [ ] Test documentation updates
- [ ] CI passing with all tests
- [ ] Coverage report generation
- [ ] Performance benchmarks

## Success Criteria
- All hooks have corresponding test files
- Tests run successfully on Ubuntu and macOS
- Code coverage >80%
- All tests pass in CI
- Clear documentation for adding new tests

## Related
- #26 - CI/CD pipeline implementation
- #22 - Nix hooks test suite

Beginne damit, die vorhandenen Tests für hooks/commits/gitlint.sh und hooks/nix/nix-fmt.sh zu lesen, untersuche anschließend die verfügbaren Test-Hilfsprogramme und führe die aktuelle Suite aus. Füge entsprechende Tests für die verbleibenden aufgeführten Hooks hinzu, die Existenz, Abhängigkeiten, Erfolg, Fehler und Randfälle abdecken. Als abgeschlossen gilt die Arbeit, wenn alle Hooks getestet sind, die Ubuntu- und macOS-CI erfolgreich sind, die Abdeckung 80 % überschreitet und die Testdokumentation sowie die Abdeckungsberichte aktualisiert wurden.

Vom Indexierungsmodell aus dem Issue-Text verfasst.