Feature: Add priority levels to Task

[ryanjmichie-git]

Tasks need a priority so users can triage their lists.

Requirements:

• Each Task has a priority attribute with allowed values: low, medium, high. Default is medium.
• Creating or setting a Task with any other priority value raises ValueError.
• TaskStore gains a filter_by_priority(priority) method returning tasks matching the given priority.
• Existing behavior (creation, completion, serialization) is unchanged for tasks that never set a priority.

This is a v1.1.0 hands-on test issue for ForgeProof.