docs: remove plenary from install dependencies

[fengjikui]

Summary

• remove nvim-lua/plenary.nvim from the lazy.nvim installation example because the plugin runtime no longer requires it
• remove the redundant CI step that installed plenary globally; the test init still provisions the test harness under deps/ when tests run

Closes #394 for the user-facing/runtime dependency path while keeping the current test harness unchanged.

Validation

• git diff --check
• rg -n "nvim-lua/plenary.nvim|Install plenary|plenary.nvim" README.md .github/workflows/tests.yml lua plugin ftplugin (no matches)
• Could not run ./run_tests.sh -t minimal locally because this machine does not have nvim installed; the command exits before tests with nvim: command not found.

[sudo-tee]

Thanks for the PR.

I will have a look to ensure everything is fine. But at first everything seems fine. The deps are indeed only needed and provided by the test harness.

[sudo-tee]

All seem to work fine in CI and on local.

Thanks for the cleanup