docs: add PMEDIA Agent Pet MVP roadmap - #1

Summary

Adds the first PMEDIA Agent Pet MVP foundation on top of the OpenPets fork.

What changed

• Added PMEDIA Agent Pet MVP roadmap.
• Added hybrid PMEDIA Agent Notification Protocol design:
  • Direct Generic API Sources;
  • PMEDIA Notification Gateway;
  • Special Adapters such as GitHub.
• Added dev plugin pmedia.notifications under plugins/dev.
• Added sample notification renderer with severity mapping, alert bubble, OS notification, sound, action URL, and dedupe storage.
• Added plugin harness tests.
• Added GitHub Actions desktop build workflow.

Why

The goal is not to build for one backend such as attendance only. The app should become a generic desktop notification client for PMEDIA systems, while still allowing dedicated adapters for external platforms like GitHub.

Current MVP state

This PR proves local rendering first. It does not yet connect to real backend APIs.

Next steps:

1. Run the fork locally.
2. Load plugins/dev/pmedia.notifications in Developer Mode.
3. Test Test PMEDIA notification and Test warning notification from the pet menu.
4. Add source configuration page.
5. Add direct generic API polling.
6. Add PMEDIA Notification Gateway support.

Scope

Runtime core is intentionally untouched for now. The first implementation is isolated as a dev plugin.