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/note.py

@@ -0,0 +1,22 @@
+"""Note model definition."""
+from enum import Enum
+from pydantic import BaseModel, Field
+from datetime import datetime
+
+
+class NoteType(Enum):
+    """Types of notes that can be recorded during a session."""
+    WORK = "work"
+    CHANGE = "change"
+    BLOCKER = "blocker"
+    DECISION = "decision"
+    IDEA = "idea"
+    REFERENCE = "reference"
+
+
+class Note(BaseModel):
+    """Represents a note recorded during a session."""
+
+    type: NoteType
+    text: str
+    created_at: datetime = Field(default_factory=datetime.now)