feat: add wiki build kind with file-structure-derived navigation

- add build_wiki_nav deriving MkDocs nav from docs/wiki file structure
  (index.md -> Home, numeric prefixes stripped and title-cased, nested
  dirs become groups, natural ordering)
- add --wiki / --wiki-dir to build; wiki-only builds need no --module
- merge wiki nav before generated lib/api nav; wiki Home replaces the
  nav spec Home entry
- add mkdocs.wiki.yml template fragment and nav/cli tests
- dogfood doc-forge's own docs/wiki and regenerate site output

docforge/cli/mkdocs_utils.pyi

@@ -17,6 +17,7 @@ def generate_config(
     modes: Iterable[str] | None = None,
     site_description: str | None = None,
     site_author: str | None = None,
+    wiki_dir: Path | None = None,
 ) -> None: ...
 def build(mkdocs_yml: Path) -> None: ...
 def serve(mkdocs_yml: Path) -> None: ...