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/adapters/__init__.py

@@ -0,0 +1,28 @@
+"""
+Mail provider adapter implementations for Mail Intake.
+
+This package contains **adapter-layer implementations** responsible for
+interfacing with external mail providers and exposing a normalized,
+provider-agnostic contract to the rest of the system.
+
+Adapters in this package:
+- Implement the `MailIntakeAdapter` interface
+- Encapsulate all provider-specific APIs and semantics
+- Perform read-only access to mail data
+- Return provider-native payloads without interpretation
+
+Provider-specific logic **must not leak** outside of adapter implementations.
+All parsings, normalizations, and transformations must be handled by downstream
+components.
+
+Public adapters exported from this package are considered the supported
+integration surface for mail providers.
+"""
+
+from .base import MailIntakeAdapter
+from .gmail import MailIntakeGmailAdapter
+
+__all__ = [
+    "MailIntakeAdapter",
+    "MailIntakeGmailAdapter",
+]