Align example name with Agent Plugins project family - #1

Summary

• rename the example identity from agent-plugin-example to agent-plugins-example
• align the README title and illustrated package root with the repository name
• align the portable manifest name with the canonical example directory

Why

The repository is part of the Agent Plugins project family alongside agent-plugins-spec and agent-plugins-site. Using the shared plural prefix makes that relationship visible and lets the canonical example continue modeling matching directory and manifest names.

Impact

This is a naming-only change. It does not change plugin behavior or rewrite the contributor's existing history.

Validation

• parsed plugin.json with jq
• verified the manifest name is exactly agent-plugins-example
• confirmed no stale singular example identity remains in the changed surfaces
• ran git diff --check