docs(mail_intake): add comprehensive docstrings across ingestion, adapters, auth, and parsing layers

- docs(mail_intake/__init__.py): document module-based public API and usage patterns
- docs(mail_intake/ingestion/reader.py): document high-level ingestion orchestration
- docs(mail_intake/adapters/base.py): document adapter contract for mail providers
- docs(mail_intake/adapters/gmail.py): document Gmail adapter implementation and constraints
- docs(mail_intake/auth/base.py): document authentication provider contract
- docs(mail_intake/auth/google.py): document Google OAuth authentication provider
- docs(mail_intake/models/message.py): document canonical email message model
- docs(mail_intake/models/thread.py): document canonical email thread model
- docs(mail_intake/parsers/body.py): document message body extraction logic
- docs(mail_intake/parsers/headers.py): document message header normalization utilities
- docs(mail_intake/parsers/subject.py): document subject normalization utilities
- docs(mail_intake/config.py): document global configuration model
- docs(mail_intake/exceptions.py): document library exception hierarchy

mail_intake/models/__init__.py

@@ -0,0 +1,22 @@
+"""
+Domain models for Mail Intake.
+
+This package defines the **canonical, provider-agnostic data models**
+used throughout the Mail Intake ingestion pipeline.
+
+Models in this package:
+- Represent fully parsed and normalized mail data
+- Are safe to persist, serialize, and index
+- Contain no provider-specific payloads or API semantics
+- Serve as stable inputs for downstream processing and analysis
+
+These models form the core internal data contract of the library.
+"""
+
+from .message import MailIntakeMessage
+from .thread import MailIntakeThread
+
+__all__ = [
+    "MailIntakeMessage",
+    "MailIntakeThread",
+]