Add skill to facilitate SPFx version upgrades

Summary

Adding support for a new SPFx version (e.g. spfx project upgrade, spfx project doctor, spfx doctor) is a recurring, well-structured task that follows the same pattern each time. A Claude Code skill would allow contributors to execute this workflow consistently and efficiently without having to rediscover the steps.

What the skill would cover

• Researching the new version (release notes, what changed, skipped/delisted versions)
• Creating upgrade-{version}.ts and doctor-{version}.ts
• Updating supportedVersions in project-upgrade.ts and project-doctor.ts
• Adding the entry to SpfxCompatibilityMatrix.ts for spfx doctor
• Guiding the contributor through manual test project scaffolding with yo @microsoft/sharepoint (including which project types to create and how)
• Adding e2e tests with correct finding counts (determined by running the test suite)
• Validating finding counts against equivalent previous patch upgrades for consistency
• Updating docs and opening the PR with a standardized description

Why a skill

The workflow has a natural human-in-the-loop step (scaffolding test projects requires the correct generator version installed locally), making it a good fit for a skill that can pause, hand off, and resume. The code changes themselves are mechanical and well-suited for AI assistance.