docs: add wiki and refresh flat lib with mcp artifacts

- Add hand-written wiki (index, overview, how-to, extending, dev)
  following the platform anatomy
- Remove stale nested docs/lib/mail_intake and regenerate the flat
  lib reference to match docforge.nav.yml
- Regenerate MCP bundle with standardized docstrings

docs/mcp/index.json

@@ -1,5 +1,5 @@
 {
-  "project": "mail_intake",
+  "project": "Mail Intake",
   "type": "docforge-model",
   "modules_count": 22,
   "source": "docforge"