Implement storage layer for MVP-1 Personal Tracker CLI

Add storage layer with FileStorage, MarkdownReader, and MarkdownWriter classes.
Add data models (Project, Session, Note, Change).

tracker/models/__init__.py

@@ -0,0 +1,7 @@
+"""Data models for the tracker."""
+from .project import Project
+from .session import Session
+from .note import Note, NoteType
+from .change import Change
+
+__all__ = ["Project", "Session", "Note", "NoteType", "Change"]