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/parsers/__init__.py

@@ -0,0 +1,30 @@
+"""
+Message parsing utilities for Mail Intake.
+
+This package contains **provider-aware but adapter-agnostic parsing helpers**
+used to extract and normalize structured information from raw mail payloads.
+
+Parsers in this package are responsible for:
+- Interpreting provider-native message structures
+- Extracting meaningful fields such as headers, body text, and subjects
+- Normalizing data into consistent internal representations
+
+This package does not:
+- Perform network or IO operations
+- Contain provider API logic
+- Construct domain models directly
+
+Parsing functions are designed to be composable and are orchestrated by the
+ingestion layer.
+"""
+
+from .body import extract_body
+from .headers import parse_headers, extract_sender
+from .subject import normalize_subject
+
+__all__ = [
+    "extract_body",
+    "parse_headers",
+    "extract_sender",
+    "normalize_subject",
+]