Add Vardoger plugin

[dstrupl]

What

Add Vardoger 0.3.2 as an externally hosted plugin in the default Copilot
plugins marketplace and include it in the external-plugins README section.

Why

Vardoger packages a Copilot skill that turns locally stored conversation
history into tailored user or project instructions. The runtime is
open-source, has no hosted backend or telemetry, and is already installable
from its own repository; this entry makes it discoverable from the default
marketplace.

User impact

Users can discover and install the vardoger plugin while the marketplace
continues to resolve its source from dstrupl/vardoger:plugins/copilot.

Checks

• Parsed .github/plugin/marketplace.json with jq.
• Confirmed the .claude-plugin/marketplace.json compatibility symlink resolves
  to the same manifest.
• Confirmed the source plugin manifest declares Vardoger 0.3.2 and its
  skills/ directory.
• Ran git diff --check.