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/thread.py

@@ -1,3 +1,13 @@
+"""
+Thread domain models for Mail Intake.
+
+This module defines the **canonical, provider-agnostic representation**
+of an email thread as used internally by the Mail Intake ingestion pipeline.
+
+Threads group related messages and serve as the primary unit of reasoning
+for higher-level correspondence workflows.
+"""
+
 from dataclasses import dataclass, field
 from datetime import datetime
 from typing import List, Set
@@ -10,21 +20,40 @@ class MailIntakeThread:
     """
     Canonical internal representation of an email thread.
 
-    Threads are the primary unit of reasoning for correspondence
-    workflows (job applications, interviews, follow-ups, etc.).
+    A thread groups multiple related messages under a single subject
+    and participant set. It is designed to support reasoning over
+    conversational context such as job applications, interviews,
+    follow-ups, and ongoing discussions.
+
+    This model is provider-agnostic and safe to persist.
     """
 
     thread_id: str
+    """Provider-specific thread identifier."""
+
     normalized_subject: str
+    """Normalized subject line used to group related messages."""
 
     participants: Set[str] = field(default_factory=set)
+    """Set of unique participant email addresses observed in the thread."""
+
     messages: List[MailIntakeMessage] = field(default_factory=list)
+    """Ordered list of messages belonging to this thread."""
 
     last_activity_at: datetime | None = None
+    """Timestamp of the most recent message in the thread."""
 
     def add_message(self, message: MailIntakeMessage) -> None:
         """
         Add a message to the thread and update derived fields.
+
+        This method:
+        - Appends the message to the thread
+        - Tracks unique participants
+        - Updates the last activity timestamp
+
+        Args:
+            message: Parsed mail message to add to the thread.
         """
         self.messages.append(message)