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/services/session_service.py

@@ -0,0 +1,67 @@
+"""Session service for active session management."""
+
+import json
+from datetime import datetime
+from pathlib import Path
+from typing import Optional
+
+from ..models import Session
+
+
+_ACTIVE_SESSION_FILE = ".active_session.json"
+
+
+def get_active_session_path() -> Path:
+    """Return the path to the active session file in projects/ directory."""
+    return Path("projects") / _ACTIVE_SESSION_FILE
+
+
+def get_active_session() -> Optional[Session]:
+    """Load and return the currently active session, or None if none exists."""
+    path = get_active_session_path()
+    if not path.exists():
+        return None
+
+    with open(path, "r", encoding="utf-8") as f:
+        data = json.load(f)
+
+    # Convert started_at string back to datetime
+    data["started_at"] = datetime.fromisoformat(data["started_at"])
+    if data.get("ended_at"):
+        data["ended_at"] = datetime.fromisoformat(data["ended_at"])
+
+    return Session(**data)
+
+
+def set_active_session(session: Session) -> None:
+    """Save the given session as the active session."""
+    path = get_active_session_path()
+    path.parent.mkdir(parents=True, exist_ok=True)
+
+    data = session.model_dump(mode="json")
+    # Serialize datetime objects to ISO format
+    data["started_at"] = session.started_at.isoformat()
+    if session.ended_at:
+        data["ended_at"] = session.ended_at.isoformat()
+
+    with open(path, "w", encoding="utf-8") as f:
+        json.dump(data, f, indent=2, ensure_ascii=False)
+
+
+def clear_active_session() -> None:
+    """Remove the active session file."""
+    path = get_active_session_path()
+    if path.exists():
+        path.unlink()
+
+
+def validate_no_other_active_session(project_slug: str) -> bool:
+    """
+    Check if there is an active session for a different project.
+    Returns True if no conflict exists (i.e., either no active session
+    or the active session belongs to the same project).
+    """
+    active = get_active_session()
+    if active is None:
+        return True
+    return active.project_slug == project_slug