Loads a checkpoint by ID. Returns undefined if not found.
Loads the latest checkpoint for a given execution ID.
Lists all checkpoint summaries for a given execution ID.
Deletes a checkpoint by ID. Returns true if found and deleted.
Deletes all checkpoints for a given execution ID.
Returns total number of checkpoints across all executions.
Clears all checkpoints.
Abstract checkpoint store interface. Implementations provide persistence (in-memory, JSON file, SQLite, etc.).