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/04_iterative_workflow.md

@@ -0,0 +1,33 @@
+# Iterative Workflow
+
+A docs build runs through the CLI in a single pass.
+
+## Build commands
+
+```bash
+# Library reference only
+doc-forge build --mkdocs --module docforge
+
+# Wik + library combined (single MkDocs build)
+doc-forge build --wiki --mkdocs --module docforge
+
+# Wiki only — no module required
+doc-forge build --wiki --site-name docforge
+```
+
+## What a combined build does
+
+1. Validates the requested modes (`--mkdocs`, `--api`, `--wiki`, `--mcp`).
+2. Generates library sources under `docs/lib/**` with `MkDocsRenderer`.
+3. Generates API sources under `docs/api/**` when `--api` is given.
+4. Derives wiki navigation from `docs/wiki/**`.
+5. Writes `mkdocs.yml` with merged navigation — wiki first, generated groups
+   appended, and the wiki `Home` replacing any spec `Home` entry.
+6. Runs `mkdocs build` once and emits the site.
+
+## Explore the site
+
+```bash
+doc-forge build --wiki --mkdocs --module docforge
+doc-forge serve --mkdocs-yml mkdocs.yml
+```