Add plugin manifest validation guardrail

[rogermt]

Goal

Add a GitHub Actions workflow that validates plugin manifests on every PR.

Why

• Prevents half-migrated plugins
• Catches manifest regressions early
• Lightweight: uses curl + jq, no test framework required
• Enforces: /v1/plugins endpoint works, every plugin exposes /v1/plugins/{plugin}/manifest

Changes

• Add .github/workflows/plugin-manifest-guardrail.yml
• Validates on every push to main and all PRs

Testing

CI will run this workflow on the PR that implements it.