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/modules/mail_intake.config.json

@@ -4,18 +4,11 @@
     "path": "mail_intake.config",
     "docstring": "# Summary\n\nGlobal configuration models for Mail Intake.\n\nThis module defines the **top-level configuration object** used to control\nmail ingestion behavior across adapters, authentication providers, and\ningestion workflows.\n\nConfiguration is intentionally explicit, immutable, and free of implicit\nenvironment reads to ensure predictability and testability.",
     "objects": {
-      "dataclass": {
-        "name": "dataclass",
-        "kind": "alias",
-        "path": "mail_intake.config.dataclass",
-        "signature": "<bound method Alias.signature of Alias('dataclass', 'dataclasses.dataclass')>",
-        "docstring": null
-      },
       "MailIntakeConfig": {
         "name": "MailIntakeConfig",
         "kind": "class",
         "path": "mail_intake.config.MailIntakeConfig",
-        "signature": "<bound method Class.signature of Class('MailIntakeConfig', 17, 56)>",
+        "signature": "MailIntakeConfig(provider: str = ..., user_id: str = ..., readonly: bool = ..., credentials_path: str | None = ..., token_path: str | None = ...)",
         "docstring": "Global configuration for `mail-intake`.\n\nNotes:\n    **Guarantees:**\n\n        - This configuration is intentionally explicit and immutable.\n        - No implicit environment reads or global state.\n        - Explicit configuration over implicit defaults.\n        - No direct environment or filesystem access.\n        - This model is safe to pass across layers and suitable for\n          serialization.",
         "members": {
           "provider": {