feat: add reusable-metrics workflow

[nearlyforget]

Description

This pull request implements a reusable GitHub Actions workflow for generating weekly metrics. This workflow automates the collection of statistics for Issues, Pull
Requests, and Discussions, and aggregates them into a consolidated report.

The report is then automatically posted as a new issue in the calling repository, providing maintainers with a clear overview of the previous week's activity.

Category (Required)

• Infrastructure: CI/CD, Linters, or build scripts.
• Documentation: Maturity levels and reporting.

Related Issues

N/A

Checklist

• x] I have followed the [Contributing Guide (https://github.com/agentic-commerce/ucp/blob/main/CONTRIBUTING.md).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.

Screenshots / Logs (if applicable)