docs(api): update package docs for credential store–based auth design

- Update package-level documentation to reflect credential persistence
  as a first-class concept
- Replace token_path-based examples with CredentialStore usage
- Add credentials module to documented public API surface
- Clarify auth vs credential persistence responsibilities
- Align design guarantees with distributed-safe authentication model

mail_intake/__init__.py

@@ -9,7 +9,8 @@ The library is intentionally structured around clear layers, each exposed
 as a first-class module at the package root:
 
 - adapters: provider-specific access (e.g. Gmail)
-- auth: authentication providers and credential management
+- auth: authentication providers and credential lifecycle management
+- credentials: credential persistence abstractions and implementations
 - parsers: extraction and normalization of message content
 - ingestion: orchestration and high-level ingestion workflows
 - models: canonical, provider-agnostic data representations
@@ -38,15 +39,18 @@ required by the selected provider (for example, Google APIs for Gmail).
 Basic Usage
 ----------------------------------------------------------------------
 
-Minimal Gmail ingestion example:
+Minimal Gmail ingestion example (local development):
 
     from mail_intake.ingestion import MailIntakeReader
     from mail_intake.adapters import MailIntakeGmailAdapter
     from mail_intake.auth import MailIntakeGoogleAuth
+    from mail_intake.credentials import PickleCredentialStore
+
+    store = PickleCredentialStore(path="token.pickle")
 
     auth = MailIntakeGoogleAuth(
         credentials_path="credentials.json",
-        token_path="token.pickle",
+        store=store,
         scopes=["https://www.googleapis.com/auth/gmail.readonly"],
     )
 
@@ -69,9 +73,11 @@ Mail Intake is designed to be extensible via **public contracts** exposed
 through its modules:
 
 - Users MAY implement their own mail adapters by subclassing
-  `adapters.MailIntakeAdapter`
+  ``adapters.MailIntakeAdapter``
 - Users MAY implement their own authentication providers by subclassing
-  `auth.MailIntakeAuthProvider`
+  ``auth.MailIntakeAuthProvider[T]``
+- Users MAY implement their own credential persistence layers by
+  implementing ``credentials.CredentialStore[T]``
 
 Users SHOULD NOT subclass built-in adapter implementations. Built-in
 adapters (such as Gmail) are reference implementations and may change
@@ -86,6 +92,7 @@ The supported public API consists of the following top-level modules:
 - mail_intake.ingestion
 - mail_intake.adapters
 - mail_intake.auth
+- mail_intake.credentials
 - mail_intake.parsers
 - mail_intake.models
 - mail_intake.config
@@ -103,6 +110,7 @@ Design Guarantees
 - Explicit configuration and dependency injection
 - No implicit global state or environment reads
 - Deterministic, testable behavior
+- Distributed-safe authentication design
 
 Mail Intake favors correctness, clarity, and explicitness over convenience
 shortcuts.
@@ -112,6 +120,7 @@ shortcuts.
 from . import ingestion
 from . import adapters
 from . import auth
+from . import credentials
 from . import models
 from . import config
 from . import exceptions
@@ -120,6 +129,7 @@ __all__ = [
     "ingestion",
     "adapters",
     "auth",
+    "credentials",
     "models",
     "config",
     "exceptions",