Core: Fix staticCopy not copying index.html to sub directory

[ndelangen]

What I did

When a user adds a static assets directory without a to location, it's assumed to be at the top level.
In that case, adding index.html and iframe.html are not allowed, because they'd break the static storybook.

So in the past we added some code to prevent those files from getting copied.

However I discovered a bug, where copying to a-sub-directory/index.html was also blocked.

This PR fixes that bug.

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

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

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

• 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.

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

[nx-cloud]

View your CI Pipeline Execution ↗ for commit 32046f0

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☁️ Nx Cloud last updated this comment at 2025-08-13 20:10:04 UTC

[storybook-app-bot]

Package Benchmarks

^{Commit: 32046f0, ran on 13 August 2025 at 20:03:27 UTC}

The following packages have significant changes to their size or dependencies:

@storybook/cli

	Before	After	Difference
Dependency count	220	219	🎉 -1 🎉
Self size	878 KB	878 KB	🚨 +90 B 🚨
Dependency size	83.28 MB	81.87 MB	🎉 -1.41 MB 🎉
Bundle Size Analyzer	Link	Link

@storybook/codemod

	Before	After	Difference
Dependency count	189	188	🎉 -1 🎉
Self size	34 KB	35 KB	🚨 +42 B 🚨
Dependency size	78.36 MB	76.95 MB	🎉 -1.41 MB 🎉
Bundle Size Analyzer	Link	Link

create-storybook

	Before	After	Difference
Dependency count	50	49	🎉 -1 🎉
Self size	1.52 MB	1.52 MB	0 B
Dependency size	49.72 MB	48.32 MB	🎉 -1.41 MB 🎉
Bundle Size Analyzer	node	node

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