chore(release): 0.0.3

Squash-merge lib-standardisation: adopt repo standards across packaging,
tooling, docs, and CI.

- standardize pyproject.toml (canonical packaging, lint config, extras, GSDFC pydoclint)
- add py.typed marker, modernize type hints and Google-style docstrings
- nest generated docs under docs/lib and docs/mcp; add hand-written wiki
- adopt standard Type A PyPI CI pipeline (tag==version guard + quality gate)
- promote generated CHANGELOG.md; bump version to 0.0.3

mail_intake/credentials/store.pyi

@@ -1,11 +1,11 @@
 from abc import ABC, abstractmethod
-from typing import Generic, Optional, TypeVar
+from typing import Generic, TypeVar
 
 T = TypeVar("T")
 
 class CredentialStore(ABC, Generic[T]):
     @abstractmethod
-    def load(self) -> Optional[T]: ...
+    def load(self) -> T | None: ...
     @abstractmethod
     def save(self, credentials: T) -> None: ...
     @abstractmethod