Release v3.2.0

fix(plugin): sync plugin manifests to 3.2.0 and gate it locally

The v3 line bumped package.json four times without updating the plugin
manifests, so plugin.json and marketplace.json were still on 2.13.0 and
plugin/.mcp.json still pinned 3.1.0.

Root cause is not the drift, it is that `check:plugin` ran only in CI and not in
`npm run validate`. Every increment of the v3 line reported a green validate
locally while carrying a failure that could only surface after a push. Adding it
to validate makes the local command mean what its name claims.

npm run validate: green, 629 tests, plugin in sync at 3.2.0.