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.exceptions.json

@@ -8,28 +8,28 @@
         "name": "MailIntakeError",
         "kind": "class",
         "path": "mail_intake.exceptions.MailIntakeError",
-        "signature": "<bound method Class.signature of Class('MailIntakeError', 15, 25)>",
+        "signature": null,
         "docstring": "Base exception for all Mail Intake errors.\n\nNotes:\n    **Guarantees:**\n\n        - This is the root of the Mail Intake exception hierarchy\n        - All errors raised by the library must derive from this class\n        - Consumers should generally catch this type when handling library-level failures"
       },
       "MailIntakeAuthError": {
         "name": "MailIntakeAuthError",
         "kind": "class",
         "path": "mail_intake.exceptions.MailIntakeAuthError",
-        "signature": "<bound method Class.signature of Class('MailIntakeAuthError', 28, 37)>",
+        "signature": null,
         "docstring": "Authentication and credential-related failures.\n\nNotes:\n    **Lifecycle:**\n\n        - Raised when authentication providers are unable to acquire,\n          refresh, or persist valid credentials."
       },
       "MailIntakeAdapterError": {
         "name": "MailIntakeAdapterError",
         "kind": "class",
         "path": "mail_intake.exceptions.MailIntakeAdapterError",
-        "signature": "<bound method Class.signature of Class('MailIntakeAdapterError', 40, 49)>",
+        "signature": null,
         "docstring": "Errors raised by mail provider adapters.\n\nNotes:\n    **Lifecycle:**\n\n        - Raised when a provider adapter encounters API errors, transport\n          failures, or invalid provider responses."
       },
       "MailIntakeParsingError": {
         "name": "MailIntakeParsingError",
         "kind": "class",
         "path": "mail_intake.exceptions.MailIntakeParsingError",
-        "signature": "<bound method Class.signature of Class('MailIntakeParsingError', 52, 61)>",
+        "signature": null,
         "docstring": "Errors encountered while parsing message content.\n\nNotes:\n    **Lifecycle:**\n\n        - Raised when raw provider payloads cannot be interpreted or\n          normalized into internal domain models."
       }
     }