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.models.message.json

@@ -4,25 +4,11 @@
     "path": "mail_intake.models.message",
     "docstring": "# Summary\n\nMessage domain models for Mail Intake.\n\nThis module defines the **canonical, provider-agnostic representation**\nof an individual email message as used internally by the Mail Intake\ningestion pipeline.\n\nModels in this module are safe to persist and must not contain any\nprovider-specific fields or semantics.",
     "objects": {
-      "dataclass": {
-        "name": "dataclass",
-        "kind": "alias",
-        "path": "mail_intake.models.message.dataclass",
-        "signature": "<bound method Alias.signature of Alias('dataclass', 'dataclasses.dataclass')>",
-        "docstring": null
-      },
-      "datetime": {
-        "name": "datetime",
-        "kind": "alias",
-        "path": "mail_intake.models.message.datetime",
-        "signature": "<bound method Alias.signature of Alias('datetime', 'datetime.datetime')>",
-        "docstring": null
-      },
       "MailIntakeMessage": {
         "name": "MailIntakeMessage",
         "kind": "class",
         "path": "mail_intake.models.message.MailIntakeMessage",
-        "signature": "<bound method Class.signature of Class('MailIntakeMessage', 18, 79)>",
+        "signature": "MailIntakeMessage(message_id: str, thread_id: str, timestamp: datetime, from_email: str, from_name: str | None, subject: str, body_text: str, snippet: str, raw_headers: dict[str, str])",
         "docstring": "Canonical internal representation of a single email message.\n\nNotes:\n    **Guarantees:**\n\n        - This model represents a fully parsed and normalized email message.\n        - It is intentionally provider-agnostic and suitable for\n          persistence, indexing, and downstream processing.\n\n    **Constraints:**\n\n        - No provider-specific identifiers, payloads, or API semantics\n          should appear in this model.",
         "members": {
           "message_id": {