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/wiki/05_development/02_quality_gates.md

@@ -0,0 +1,22 @@
+# Quality Gates
+
+Run all checks before pushing:
+
+```bash
+.venv\Scripts\python.exe -m pytest
+.venv\Scripts\python.exe -m ruff check docforge tests
+.venv\Scripts\python.exe -m black --check docforge tests
+.venv\Scripts\python.exe -m mypy docforge
+```
+
+## Test layout
+
+| Path                     | Covers                             |
+|--------------------------|------------------------------------|
+| `tests/nav/`             | Nav spec, resolver, wiki nav       |
+| `tests/cli/`             | Build command flows and modes      |
+| `tests/renderers/`       | MkDocs / MCP rendering             |
+
+CLI tests use the `cli_runner` fixture with `mock_mkdocs_build` and
+`mock_mkdocs_load_config` so they exercise the full flow without invoking a
+real MkDocs build.