This massive update increases test coverage from 213 to 285 tests (+72 tests, +34%)
to prepare for the gh-pages v6 upgrade (PR #186). All critical paths are now tested,
prepareOptions is refactored into testable functions, and a public API is exported.

## Phase 1: Critical Test Gaps (20 new tests)

### gh-pages Error Callback Tests (4 tests)
- Add tests for the "do NOT await" error handling pattern
- Test error rejection, message preservation, and auth failures
- Verifies we never fall into gh-pages v3 silent failure trap
- File: src/engine/engine.spec.ts

### Monkeypatch Verification Tests (5 tests)
- Add first-ever tests for util.debuglog interception (was 0% coverage)
- Test gh-pages logging forwarding to Angular logger
- Test message formatting with placeholders
- Verify critical ordering (monkeypatch before gh-pages require)
- File: src/engine/engine.spec.ts

### Real Filesystem Tests (11 tests)
- Create first-ever real filesystem integration tests
- Use actual temp directories instead of mocks
- Test .nojekyll, CNAME, and 404.html file creation
- Test graceful error handling when index.html missing
- File: src/engine/engine-filesystem.spec.ts (NEW)

## Phase 2: Refactor prepareOptions (34 new tests)

### Code Refactoring
- Extract prepareOptions (157 lines) into 6 testable functions:
  1. setupMonkeypatch() - Intercept util.debuglog
  2. mapNegatedBooleans() - Transform noDotfiles → dotfiles
  3. handleUserCredentials() - Create user object or warn
  4. warnDeprecatedParameters() - Warn about deprecated noSilent
  5. appendCIMetadata() - Add CI environment metadata
  6. injectTokenIntoRepoUrl() - Inject authentication tokens
- prepareOptions() is now a clean orchestrator function
- File: src/engine/engine.ts

### Intensive Tests
- Add 34 comprehensive tests covering 100% of option transformation logic
- Test every extracted function exhaustively with all edge cases
- Covers all boolean combinations, CI environments, and token scenarios
- File: src/engine/prepare-options.spec.ts (NEW)

## Phase 3: Export Public API (18 new tests)

### Public API Exports
- Export Schema, PublishOptions, DeployUser, and other types
- Export defaults configuration object
- Export core functions (deployToGHPages, angularDeploy)
- Export advanced functions (all prepareOptions helpers)
- Users can now extend angular-cli-ghpages functionality
- File: src/public_api.ts

### Public API Tests
- Add 18 tests verifying all exports are accessible
- Test type compilation and runtime function availability
- Include example usage scenarios
- File: src/public-api.spec.ts (NEW)

## Documentation

- Add TEST_COVERAGE_PLAN.md documenting the complete plan
- Includes implementation order, success criteria, and file listing
- Documents context from PR #186 analysis

## Test Results

- Test suites: 16 → 18 (+2)
- Tests: 213 → 285 (+72, +34%)
- Pass rate: 100% (285/285)
- Zero regressions

## Breaking Changes

None - all changes are backwards compatible. The refactoring maintains
exact same behavior while adding exports and improving testability.

## Related

- Addresses PR #186 preparation (gh-pages v3 → v6 upgrade)
- Completes audit remediation priorities 7-10
- Enables future refactoring with test safety net