Add a repository-scoped Microsoft writing style skill

[MSBrett]

⚠️ Problem

Repository agents don't have a shared, repository-scoped implementation of the Microsoft Writing Style Guide. Contributors must repeatedly supply writing guidance, and Claude Code and GitHub Copilot CLI can produce inconsistent customer-facing content.

🛠️ Solution

Vendor the microsoft-writing-style-guide Agent Skill under .claude/skills so both Claude Code and GitHub Copilot CLI can discover it. Install it from msbrettorg/microsoft-writing-style, pin it to a full commit SHA, and retain GitHub CLI provenance metadata so updates are explicit and reproducible.

ℹ️ Additional context

The proposed snapshot comes from msbrettorg/microsoft-writing-style commit d9227efa6c984c13260e3bf869f4b6132cfb3c19. It includes an offline validator, a 870-term index, representative evaluations, source attribution, and a pinned Microsoft Writing Style Guide source manifest. A normal gh skill update skips the pinned skill; maintainers must intentionally reinstall it with a new pin to update it.

🙋‍♀️ Ask for the community

We could use your help:

1. Review whether the repository-scoped skill triggers appropriately for customer-facing content work.
2. Suggest additional representative writing scenarios that should be covered by the skill evaluations.

[microsoft-github-policy-service]

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