Update AGENTS.md: Enforce 'Ownership-on-Completion' (Assign before Close)

Goal: Ensure accurate velocity tracking and historical attribution by enforcing that tickets are assigned to the active user before being closed.

Context: Currently, tickets are often created, worked on, and closed without an assignee. This creates a gap in analytics ("who did this work?"). Since most tasks are completed in a single session (15-60m), assigning upon completion is the most accurate signal of ownership.

Changes Required:

1. Update .github/AGENTS.md (Section 6: Git Protocol).
2. Add a rule: "Ownership-on-Completion: Before closing a ticket, ensure it is assigned to the current user. If unassigned, assign it immediately to capture credit for the work."

Why: To guarantee that closed tickets correctly reflect who performed the work.