Migrate ActionList roots to mergeProps

[joshblack]

Closes #

Migrates ActionList group, heading, trailing-action, and button-item roots to the ADR-025 prop-merging convention without changing existing prop behavior.

Changelog

New

None.

Changed

None; this is a behavior-preserving refactor.

Removed

None.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

This migration preserves the existing public contract and does not require release notes.

Testing & Reviewing

Review inline class-name composition, ref ordering, and preservation of the consumer button-type override.

[changeset-bot]

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Latest commit: 1e0c5a4

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.