Proposal: Run Storybook CI on all pull requests (#6880)

## Summary
Allow the Storybook workflow to run on every PR branch instead of only
`main`.

## Changes
- **What**: remove the `branches: [main]` filter from the Storybook CI
workflow so it triggers for all pull_request events.

## Review Focus
- Confirm broader triggering is desired for all branches while Chromatic
deployment remains restricted to `version-bump-*` or manual runs.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6880-Run-Storybook-CI-on-all-pull-requests-2b56d73d36508169a32afcf50f8e8fd8)
by [Unito](https://www.unito.io)

Co-authored-by: sno <snomiao@gmail.com>

Author: benceruleanlu