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/nav/wiki.pyi

@@ -0,0 +1,15 @@
+from pathlib import Path
+from typing import Any
+
+def build_wiki_nav(wiki_dir: Path) -> list[dict[str, Any]]:
+    """
+    Derive an MkDocs navigation block from a wiki directory.
+
+    Returns:
+        Wiki navigation entries compatible with the MkDocs
+        `nav` configuration.
+
+    Raises:
+        FileNotFoundError: if the wiki directory does not exist
+    """
+    ...