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

tests/nav/test_wiki.py

@@ -0,0 +1,106 @@
+from pathlib import Path
+
+import pytest
+
+from docforge.nav import build_wiki_nav
+
+
+def _write_wiki(root: Path, files: list[str]) -> None:
+    for rel in files:
+        full = root / rel
+        full.parent.mkdir(parents=True, exist_ok=True)
+        full.write_text(f"# {rel}", encoding="utf-8")
+
+
+def test_missing_wiki_dir_raises(tmp_path: Path):
+    with pytest.raises(FileNotFoundError):
+        build_wiki_nav(tmp_path / "nope")
+
+
+def test_empty_wiki_dir_returns_empty(tmp_path: Path):
+    wiki = tmp_path / "wiki"
+    wiki.mkdir()
+
+    assert build_wiki_nav(wiki) == []
+
+
+def test_index_becomes_home(tmp_path: Path):
+    wiki = tmp_path / "docs" / "wiki"
+    _write_wiki(wiki, ["index.md", "01_overview.md"])
+
+    nav = build_wiki_nav(wiki)
+
+    assert nav[0] == {"Home": "wiki/index.md"}
+    assert {"Overview": "wiki/01_overview.md"} in nav
+
+
+def test_number_prefix_stripped_and_prettified(tmp_path: Path):
+    wiki = tmp_path / "docs" / "wiki"
+    _write_wiki(wiki, ["01_basic_crud.md", "02_custom_repo.md", "10_transactions.md"])
+
+    nav = build_wiki_nav(wiki)
+
+    labels = [list(entry)[0] for entry in nav]
+    assert labels == ["Basic Crud", "Custom Repo", "Transactions"]
+
+
+def test_directory_becomes_nested_group(tmp_path: Path):
+    wiki = tmp_path / "docs" / "wiki"
+    _write_wiki(
+        wiki,
+        [
+            "index.md",
+            "03_use_cases/01_basic_crud.md",
+            "03_use_cases/02_custom_repo.md",
+        ],
+    )
+
+    nav = build_wiki_nav(wiki)
+
+    assert {"Home": "wiki/index.md"} in nav
+    assert {
+        "Use Cases": [
+            {"Basic Crud": "wiki/03_use_cases/01_basic_crud.md"},
+            {"Custom Repo": "wiki/03_use_cases/02_custom_repo.md"},
+        ]
+    } in nav
+
+
+def test_nested_index_is_section_root(tmp_path: Path):
+    wiki = tmp_path / "docs" / "wiki"
+    _write_wiki(
+        wiki,
+        [
+            "guide/index.md",
+            "guide/01_setup.md",
+        ],
+    )
+
+    nav = build_wiki_nav(wiki)
+
+    assert {
+        "Guide": [
+            {"Guide": "wiki/guide/index.md"},
+            {"Setup": "wiki/guide/01_setup.md"},
+        ]
+    } in nav
+
+
+def test_hidden_entries_ignored(tmp_path: Path):
+    wiki = tmp_path / "docs" / "wiki"
+    _write_wiki(wiki, ["index.md", ".draft.md"])
+    (wiki / ".hidden").mkdir()
+
+    nav = build_wiki_nav(wiki)
+
+    assert nav == [{"Home": "wiki/index.md"}]
+
+
+def test_natural_order_for_zero_padded_prefixes(tmp_path: Path):
+    wiki = tmp_path / "docs" / "wiki"
+    _write_wiki(wiki, ["02_b.md", "10_j.md", "01_a.md"])
+
+    nav = build_wiki_nav(wiki)
+
+    labels = [list(entry)[0] for entry in nav if list(entry)[0] in {"A", "B", "J"}]
+    assert labels == ["A", "B", "J"]