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/commands.pyi

@@ -9,12 +9,14 @@ def build(
     mcp: bool,
     mkdocs: bool,
     api: bool,
+    wiki: bool,
     module_is_source: bool,
     module: str | None,
     openapi_spec: Path | None,
     project_name: str | None,
     site_name: str | None,
     docs_dir: Path,
+    wiki_dir: Path,
     nav_file: Path,
     template: Path | None,
     mkdocs_yml: Path,