fix: Regenerate index.yaml workflow cannot push to protected main

[scotwells]

Problem

The Regenerate index.yaml workflow (.github/workflows/generate-index.yaml) runs on push to main (paths plugins/**), regenerates index.yaml from plugins/*.yaml, and then git pushes the result to main. That push is rejected by the repository ruleset:

remote: error: GH013: Repository rule violations found for refs/heads/main.
remote: - Changes must be made through a pull request.
! [remote rejected] main -> main (push declined due to repository rule violations)

This has failed on every run since the repo was created, so index.yaml silently drifts from plugins/ whenever a plugin manifest changes — both the initial v0.8.0-dev.1 import and the v0.8.0-dev.2 bump (#2) required a manual follow-up PR (#3) to regenerate the index.

Desired outcome

Merging a plugins/*.yaml change updates the served index.yaml with no manual step. Options:

• Have the workflow open a PR with the regenerated index.yaml (e.g. peter-evans/create-pull-request) instead of pushing to main.
• Or regenerate index.yaml in validate-pr and fail the PR if it's stale, so contributors commit both plugins/*.yaml and index.yaml together.

Workaround (current)

Regenerate index.yaml locally with the workflow's script and land it via PR.