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

docs/lib/docforge/cli/index.md

@@ -0,0 +1,8 @@
+# Cli
+
+::: docforge.cli
+- [Api Utils](api_utils.md)
+- [Commands](commands.md)
+- [Main](main.md)
+- [Mcp Utils](mcp_utils.md)
+- [Mkdocs Utils](mkdocs_utils.md)