refactor: extract option helpers and add gh-pages integration tests

Extract option preparation helpers into dedicated module for better
separation of concerns and add comprehensive gh-pages integration tests.

**Refactoring:**
- Extract 6 helper functions from engine.ts to engine.prepare-options-helpers.ts:
  - setupMonkeypatch(), mapNegatedBooleans(), handleUserCredentials()
  - warnDeprecatedParameters(), appendCIMetadata(), injectTokenIntoRepoUrl()
- Create PreparedOptions type to replace verbose inline type definitions
- Move all 34 option helper tests to engine.prepare-options-helpers.spec.ts
- Reduce engine.ts from 439 to 244 lines (-44%)

**gh-pages Integration:**
- Add 30 new tests in engine.gh-pages-integration.spec.ts
- Verify exact arguments passed to gh-pages.publish() and gh-pages.clean()
- Implement options whitelist to filter internal options before passing to gh-pages
- Use module-level jest.mock() to avoid test file conflicts
- Mock Git class from gh-pages/lib/git to avoid spawning actual git processes

**Options Filtering:**
Internal options now filtered out before passing to gh-pages:
- dryRun, noDotfiles, noNotfound, noNojekyll (internal flags)
- notfound, nojekyll (used for file creation only)
- cname (used for CNAME file creation only)
- name, email (transformed to user object)

Only whitelisted options passed to gh-pages:
- repo, branch, message, remote, git, add, dotfiles, user

**Test Updates:**
- Update 7 builder-integration tests to verify filtered options
- All 315 tests passing

This ensures clean API contract with gh-pages and makes future
upgrades safer by explicitly controlling what gets passed through.

Author: JohannesHoppe

.claude/settings.local.json

@@ -24,7 +24,14 @@
       "Bash(git log:*)",
       "Bash(npm test:*)",
       "Bash(gh issue view:*)",
-      "WebSearch"
+      "WebSearch",
+      "Bash(npm ls:*)",
+      "Bash(find:*)",
+      "Bash(gh pr view:*)",
+      "Bash(gh pr diff:*)",
+      "Bash(cat:*)",
+      "Bash(npm view:*)",
+      "WebFetch(domain:raw.githubusercontent.com)"
     ],
     "deny": [],
     "ask": []