Add BranchName WithTrailingAction story

[siddharthkp]

Closes #6684

Overview

Adds a feature story demonstrating the trailing action pattern (copy button) for BranchName. This story serves as a reference for the implementation work to upstream this pattern from github-ui's PullRequestBranchName.

The story demonstrates:

• Copy button with divider line
• Hover/focus states
• Copied state feedback (icon change + tooltip)
• Accessible announcements via VisuallyHidden + Announce

Changelog

New

• WithTrailingAction story in BranchName features

Changed

None

Removed

None

Rollout strategy

• None; story-only change, no component changes yet

Testing & Reviewing

View the story in Storybook: Components > BranchName > Features > With Trailing Action

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

[changeset-bot]

⚠️ No Changeset found

Latest commit: 50df3e4

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