Add PR Duplicate Check agentic workflow (#954)

* Initial plan

* feat: add PR duplicate check agentic workflow

Adds a new GitHub Agentic Workflow that triggers on pull_request
events (opened, synchronize, reopened) to detect potential duplicate
agents, instructions, skills, and workflows being contributed via PR.

When relevant files are changed, the agent compares them against
existing resources and posts a comment on the PR listing any potential
duplicates with context and a suggestion. If no relevant files are
changed or no duplicates are found, it calls noop.

- .github/workflows/pr-duplicate-check.md: source workflow (gh-aw format)
- .github/workflows/pr-duplicate-check.lock.yml: compiled workflow

Co-authored-by: aaronpowell <434140+aaronpowell@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: aaronpowell <434140+aaronpowell@users.noreply.github.com>
Co-authored-by: Aaron Powell <me@aaron-powell.com>

Author: Copilot