Add migrate-bsv skill for Bootstrap Vue migrations

What does this MR do?

Adds a Claude Code skill at .claude/skills/migrate-bsv/SKILL.md that captures the playbook for the ongoing Bootstrap Vue removal in packages/gitlab-ui.

The skill covers:

• Why the BSV fork is vendored and how we're retiring it incrementally
• Pattern A, utils: prefer native JS, otherwise extend an existing *_utils.js rather than creating a new file.
• Pattern B, component inlining: move the BX template/props/methods into the wrapping GlX component, preserve the public API, no // copied from ... comments
• Pre-edit checklist for any file importing from vendor/bootstrap-vue/
• Inventory of completed replacements (utils + components), with breaking-change notes for each migrated component

Future contributors using Claude Code will get this context loaded automatically when touching BSV-related files, and the inventory gives a single place to look up "is this symbol already replaced?" before starting new work.

Why Claude Code skill?

Claude Code skill is picked up by Opencode, but not the other way around.

Validation

1. Check out MR
2. Claude: type /migrate-BSV and use the skill to try a migration
3. OpenCode: type /skills and then select migrate-BSV and use the skill to try a migration

Screenshots or screen recordings

Integrations

• GitLab: mr_url
• CustomersDot: mr_url
• Duo UI: mr_url
• Status Page: mr_url
• Docs: mr_url
• Switchboard: mr_url

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

Toggle the acceptance checklist

Conformity

• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
  • For example, if the MR is focused on usage guidelines, addressing accessibility challenges could be added in a separate MR.
• Relevant label(s) are applied to the MR.
• The MR is added to a milestone.
• Added the ~"component:*" label(s) if applicable.
• A changeset is added when this MR will do a patch, minor, or major update. More information can be found in doc/publishing-packages.md.

Components

• GitLab UI's contributing guidelines.
• If the MR changes a component's API, integration MR(s) have been opened (see integrations above).

Documentation

• If creating a new component page from scratch, it follows the page template structure.
• Content follows the GitLab Documentation Style Guide when appropriate.
• Related pages are cross-linked, where helpful. Component pages have related components and patterns defined in their Markdown front matter.
• If embedding a Figma file, it follows the Figma embed guide.
• Review requested from any GitLab designer or directly from a maintainer or trainee maintainer.
• Maintainer review follows the Pajamas UX maintainer review checklist.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.