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/change.py

@@ -0,0 +1,13 @@
+"""Change model definition."""
+from pydantic import BaseModel, Field
+from datetime import date
+
+
+class Change(BaseModel):
+    """Represents a notable change in a project."""
+
+    date: date
+    type: str  # code, infra, config, docs, automation, decision
+    title: str
+    impact: str = ""
+    references: list[str] = Field(default_factory=list)