[material_ui] Add Material 3 Expressive migration skill

[QuncCccccc]

This PR is to add an agent skill for Material 3 Expressive component migration work in packages/material_ui.

Related to Bring Material 3 Expressive to Flutter

Pre-Review Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
• I read the [Tree Hygiene] page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran [the auto-formatter].
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I [linked to at least one issue that this PR fixes] in the description above.
• I followed [the version and CHANGELOG instructions], using [semantic versioning] and the [repository CHANGELOG style], or I have commented below to indicate which documented exception this PR falls under[^1].
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which [test exemption] this PR falls under[^1].
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request introduces a new agent skill documentation file for Material 3 Expressive migration and links it in the skills README. The review feedback correctly points out that the documentation references a non-existent packages/material_ui package instead of packages/flutter where the Material library resides, and provides suggestions to correct these references.

[gemini-code-assist]

The Flutter Material library is located in packages/flutter (specifically under packages/flutter/lib/src/material), and there is no packages/material_ui package. Consider updating this reference to packages/flutter or the Material library to ensure accuracy.

Suggested change

	- [Material 3 Expressive migration](material-3-expressive-migration/SKILL.md): guidance for migrating `packages/material_ui` components to Material 3 Expressive.
	- [Material 3 Expressive migration](material-3-expressive-migration/SKILL.md): guidance for migrating packages/flutter Material components to Material 3 Expressive.

[gemini-code-assist]

Update the description to refer to packages/flutter instead of packages/material_ui to reflect the correct package name.

Suggested change

	description: Use when migrating packages/material_ui components to Material 3 Expressive, including generated defaults, component theme opt-ins, tests, examples, and release notes.
	description: Use when migrating packages/flutter Material components to Material 3 Expressive, including generated defaults, component theme opt-ins, tests, examples, and release notes.

[gemini-code-assist]

The Material library is part of the packages/flutter package. Consider updating packages/material_ui to packages/flutter or the Material library to match the actual package structure.

Suggested change

	Use this skill for Material 3 Expressive component migration work in `packages/material_ui`.
	Use this skill for Material 3 Expressive component migration work in packages/flutter.