Add duplicate resource detector agentic workflow

Weekly scheduled workflow that scans agents, prompts, instructions, and
skills for potential duplicates based on name, description, and content
similarity. Reports findings as a GitHub issue with task list checkboxes
for review. Checks previous duplicate-review issues to exclude known
accepted pairs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: aaronpowell