Phase 7 — Task due dates and overdue filtering

[Aaqibhafeezkhan]

Goal

Extend task metadata with an optional due date and provide a focused way to identify overdue tasks while preserving existing priority, lifecycle, persistence, and CLI behavior.

Scope

• Add an optional ISO-8601 date field (YYYY-MM-DD) to tasks.
• Persist due dates while keeping legacy task records readable.
• Validate due-date input consistently.
• Allow add/update commands to set or clear a due date.
• Add an overdue filter in the domain/service layer based on a supplied reference date so the core logic remains deterministic.
• Expose due dates and overdue filtering through the CLI.
• Add automated coverage for default/legacy dates, validation, serialization, updates, and overdue filtering.

Acceptance Criteria

• Existing task files remain readable and legacy records receive no due date.
• Valid due dates survive save/load.
• Invalid due dates are rejected without corrupting task state.
• Add/update can set and clear due dates.
• Overdue filtering is deterministic for a supplied reference date.
• Existing priority and lifecycle behavior remains intact.
• Tests cover the new metadata and filtering paths.
• No GitHub Actions are introduced.

Delivery

One focused implementation PR targeting main, exactly one implementation commit. Leave the PR open for CodeRabbit review; do not merge it automatically.