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/03_conventions.md

@@ -0,0 +1,29 @@
+# Conventions
+
+## GSDFC docstrings
+
+All documented source uses the Google-Styled Doc-Forge Convention (GSDFC):
+`Args:`, `Returns:`, `Raises:`, and `Attributes:` sections with properly typed
+signatures.
+
+## `.pyi` stubs
+
+Every module ships a matching `.pyi` stub kept in sync with the `.py`
+implementation. When signatures change, update both files.
+
+## Wiki pages
+
+- Hand-written markdown lives in `docs/wiki/**` and is never generated.
+- File names use a numeric prefix: `01_overview.md`, `02_components.md`.
+- Nested directories become nested navigation groups:
+  `05_development/01_environment.md` → group *Development*.
+- `index.md` is the site `Home` at the root, and a section root inside a
+  directory.
+- Navigation labels are derived by stripping the numeric prefix and applying
+  title case on the remaining words.
+
+## Templates
+
+MkDocs config fragments live in `docforge/templates`. The `mkdocs.wiki.yml`
+fragment carries only the `search` plugin, since wiki pages contain no
+mkdocstrings directives.