[valentinpalkovic]

Closes #

What I did

storybook add and the eslint plugin setup restyle whole config files when they only add one entry.
This PR keeps each edit to the lines it changes.

• storybook add wrote recast's output as is, which collapsed grommet's multi-line addons array into one line and indented spartan's tab-indented main.ts with 8 spaces. It now formats its edit with formatExistingFile, which uses the Prettier the project installed and the project's Prettier config, and leaves the edit as printed when the project has none.
• The shared formatFileContent is not used here, because its fallback to Prettier's defaults for an .editorconfig restyled vue-dsfr's main.ts (79 lines, with default quotes and semicolons) and gutenberg's (66 lines lost wp-prettier's paren spacing).
• The eslint plugin setup inserts its import and spread as text instead of reprinting the file with recast. grommet's eslint.config.mjs changes by 3 lines instead of 219.

-  await writeFile(fileName, await formatFileContent(fileName, formatConfig(mainConfigCSFFile)));
+  await writeFile(fileName, await formatExistingFile(fileName, formatConfig(mainConfigCSFFile)));

Split out of #36476, which collected the bugs that running storybook upgrade against ten open-source Storybook 10 monorepos turned up on next.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

Caution

This section is mandatory for all contributions. If you believe no manual test is necessary, please state so explicitly. Thanks!

1. Clone https://github.com/grommet/grommet and install dependencies.
2. Run npx storybook@<canary> upgrade --yes --config-dir storybook.
3. Confirm eslint.config.mjs gains only the plugin import and one spread.
4. Confirm storybook/main.js keeps its multi-line addons array.
5. In a tab-indented project with a Prettier config, run npx storybook@<canary> add @storybook/addon-a11y and confirm git diff on main.ts shows only the added addon.

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli-storybook/src/sandbox-templates.ts

• Declare whether manual QA will be needed for this PR during the next release, through qa:needed or qa:skip

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🤖 Generated with Claude Code

[github-actions]

	Fails
🚫	PR is not labeled with one of: ["qa:needed","qa:skip","qa:success"]
🚫	This PR needs an approving review from a Storybook Core or Developer Experience team member before it can be merged. No approvals found.

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