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

Start by reading the existing tests for hooks/commits/gitlint.sh and hooks/nix/nix-fmt.sh, then inspect the available test helper utilities and run the current suite. Add corresponding tests for the remaining listed hooks, covering existence, dependencies, success, failure, and edge cases. Done means all hooks are tested, Ubuntu and macOS CI pass, coverage exceeds 80%, and test documentation and coverage reporting are updated.

Written by the indexing model from the issue text.