ci: consolidate Storybook 'starting' state string into shell script

## Background

During review of PR #11935 (consolidate PR bot comments into one unified comment), a divergence was identified in how the Storybook starting state is rendered:

- `.github/workflows/ci-tests-storybook.yaml` (and `.github/workflows/ci-tests-storybook-forks.yaml`) hardcode `'## 🎨 Storybook: 🚧 Building...'` directly in YAML.
- `scripts/cicd/pr-storybook-deploy-and-comment.sh` (line ~139) uses an `` loading spinner for the same state.

The string now lives in two places, which means they can drift independently.

## Goal

Fold the starting-state markdown string into `scripts/cicd/pr-storybook-deploy-and-comment.sh` so it is the single source of truth. The CI workflows should invoke the script (in `starting` mode) and write to `SUMMARY_FILE`, consistent with how the `completed` case already works.

Files to update:
- `scripts/cicd/pr-storybook-deploy-and-comment.sh` — ensure the `starting` path supports `SUMMARY_FILE` output
- `.github/workflows/ci-tests-storybook.yaml` — replace hardcoded YAML string with a script invocation
- `.github/workflows/ci-tests-storybook-forks.yaml` — same fix for the forks variant

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/11935
- Review comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/11935#discussion_r3262748839

Requested by @christian-byrne