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/session.py

@@ -0,0 +1,23 @@
+"""Session model definition."""
+from pydantic import BaseModel, Field
+from datetime import datetime
+from typing import Optional
+
+
+class Session(BaseModel):
+    """Represents a work session on a project."""
+
+    id: str
+    project_slug: str
+    started_at: datetime
+    ended_at: Optional[datetime] = None
+    duration_minutes: Optional[int] = None
+    objective: str = ""
+    summary: str = ""
+    work_done: list[str] = Field(default_factory=list)
+    changes: list[str] = Field(default_factory=list)
+    decisions: list[str] = Field(default_factory=list)
+    blockers: list[str] = Field(default_factory=list)
+    next_steps: list[str] = Field(default_factory=list)
+    references: list[str] = Field(default_factory=list)
+    raw_notes: list[dict] = Field(default_factory=list)  # [{"type": "work", "text": "...", "timestamp": "..."}]