fix: plugin manifest version is out of sync with package.json

[takarki]

Summary

The Codex plugin manifest version is out of sync with the application package version. This makes the repository's own skill check fail.

Reproduction

git clone https://github.com/nexu-io/codex-slides.git
cd codex-slides
pnpm install
pnpm run check:skill

Observed at commit dbc2a5992e937760e9ce8e587e11729f970881cb (2026-07-21):

AssertionError [ERR_ASSERTION]: plugin version 0.1.0+codex.20260713 must track package version 0.2.1
    at file:///.../scripts/check-browser-first-contract.mjs:40:8

package.json declares version: 0.2.1, while .codex-plugin/plugin.json declares version: 0.1.0+codex.20260713.

Expected behavior

The plugin manifest version should intentionally track the package version (or the check should document/allow the intended versioning scheme), so pnpm run check:skill passes on a clean checkout.

Thanks for open-sourcing Codex Slides.