No plugin entry in any marketplace manifest carries a licence #5

Found by tests/repo/licensing.test.ts → the SPDX id is stated everywhere a consumer looks › every plugin entry in … (3 manifests, 2 entries each).

.claude-plugin/marketplace.json, .cursor-plugin/marketplace.json and .agents/plugins/marketplace.json had no license on any plugin entry. The marketplace entry is what a consumer reads at install time, which makes it the worst place for the licence to be absent. The three Gemini extension manifests were missing it too.

Fix: "license": "Apache-2.0" on every plugin entry and every Gemini manifest, plus a test covering each.