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/cli/test_build_wiki.py

@@ -0,0 +1,137 @@
+from pathlib import Path
+
+from docforge.cli.main import cli
+
+
+def test_wiki_only_build_requires_no_module(
+    cli_runner,
+    mock_mkdocs_build,
+    mock_mkdocs_load_config,
+):
+    with cli_runner.isolated_filesystem():
+        cwd = Path.cwd()
+
+        wiki = cwd / "docs" / "wiki"
+        wiki.mkdir(parents=True)
+        (wiki / "index.md").write_text("# Home", encoding="utf-8")
+        (wiki / "01_overview.md").write_text("# Overview", encoding="utf-8")
+
+        result = cli_runner.invoke(
+            cli,
+            [
+                "build",
+                "--wiki",
+                "--site-name",
+                "Wiki Site",
+                "--mkdocs-yml",
+                "mkdocs.yml",
+            ],
+        )
+
+        assert result.exit_code == 0, result.output
+        assert mock_mkdocs_build() is True
+        assert (cwd / "mkdocs.yml").exists()
+
+        content = (cwd / "mkdocs.yml").read_text(encoding="utf-8")
+        assert "docs_dir: docs" in content
+        assert "Home: wiki/index.md" in content
+        assert "Overview: wiki/01_overview.md" in content
+
+
+def test_wiki_combined_with_mkdocs_keeps_lib_nav(
+    cli_runner,
+    mock_mkdocs_build,
+    mock_mkdocs_load_config,
+):
+    with cli_runner.isolated_filesystem():
+        cwd = Path.cwd()
+
+        pkg = cwd / "testpkg"
+        pkg.mkdir()
+        (pkg / "__init__.py").write_text("")
+        (pkg / "mod.py").write_text("def f(): ...\n")
+
+        nav_file = cwd / "docforge.nav.yml"
+        nav_file.write_text(
+            "home: lib/testpkg/index.md\ngroups: {}\n",
+            encoding="utf-8",
+        )
+
+        wiki = cwd / "docs" / "wiki"
+        wiki.mkdir(parents=True)
+        (wiki / "index.md").write_text("# Home", encoding="utf-8")
+
+        result = cli_runner.invoke(
+            cli,
+            [
+                "build",
+                "--mkdocs",
+                "--wiki",
+                "--module",
+                "testpkg",
+                "--site-name",
+                "Combined Site",
+                "--mkdocs-yml",
+                "mkdocs.yml",
+            ],
+        )
+
+        assert result.exit_code == 0, result.output
+        assert mock_mkdocs_build() is True
+        assert (cwd / "docs" / "lib" / "testpkg" / "mod.md").exists()
+
+        content = (cwd / "mkdocs.yml").read_text(encoding="utf-8")
+        assert "docs_dir: docs" in content
+        assert "Home: wiki/index.md" in content
+        # Wiki home replaces the lib home entry
+        assert "Home: lib/testpkg/index.md" not in content
+
+
+def test_wiki_without_module_flag_ok(
+    cli_runner,
+    mock_mkdocs_build,
+    mock_mkdocs_load_config,
+):
+    with cli_runner.isolated_filesystem():
+        cwd = Path.cwd()
+
+        wiki = cwd / "docs" / "wiki"
+        wiki.mkdir(parents=True)
+        (wiki / "index.md").write_text("# Home", encoding="utf-8")
+
+        result = cli_runner.invoke(
+            cli,
+            ["build", "--wiki", "--site-name", "S"],
+        )
+
+        assert result.exit_code == 0, result.output
+        assert mock_mkdocs_build() is True
+
+
+def test_missing_wiki_dir_errors(cli_runner, mock_mkdocs_build):
+    with cli_runner.isolated_filesystem():
+        cwd = Path.cwd()
+        (cwd / "docs").mkdir()
+
+        result = cli_runner.invoke(
+            cli,
+            [
+                "build",
+                "--wiki",
+                "--site-name",
+                "S",
+                "--wiki-dir",
+                "docs/wiki",
+                "--mkdocs-yml",
+                "mkdocs.yml",
+            ],
+        )
+
+        assert result.exit_code != 0
+        assert "Wiki dir not found" in result.output
+
+
+def test_no_flag_raises_usage_error(cli_runner):
+    result = cli_runner.invoke(cli, ["build"])
+    assert result.exit_code != 0
+    assert "--mcp, --mkdocs, --wiki, or --api" in result.output