feat(agent): add rest api skill

[ashwinvaidya17]

📝 Description

• Add REST API skill

✨ Changes

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• 🚀 New feature (non-breaking change which adds functionality)
• 🐞 Bug fix (non-breaking change which fixes an issue)
• 🔄 Refactor (non-breaking change which refactors the code base)
• ⚡ Performance improvements
• 🎨 Style changes (code style/formatting)
• 🧪 Tests (adding/modifying tests)
• 📚 Documentation update
• 📦 Build system changes
• 🚧 CI/CD configuration
• 🔧 Chore (general maintenance)
• 🔒 Security update
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)

✅ Checklist

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• 📚 I have made the necessary updates to the documentation (if applicable).
• 🧪 I have written tests that support my changes and prove that my fix is effective or my feature works (if applicable).
• 🏷️ My PR title follows conventional commit format.

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[Copilot]

Pull request overview

Adds a new agent “skill” focused on REST API design/review for the Anomalib Studio FastAPI backend, and wires it into the main Copilot review priorities so it can be referenced during PR reviews affecting application/backend/.

Changes:

• Introduces a new FastAPI REST API design skill with rules and a review workflow.
• Adds a REST API checklist document to support consistent endpoint reviews.
• Updates .github/copilot-instructions.md to include the new priority section and skill reference entry.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
.github/copilot-instructions.md	Adds Priority 8 guidance for Studio backend REST API design and references the new skill in the authoritative list.
.agents/skills/fastapi-rest-api-design/SKILL.md	Defines the REST API design skill rules (routes, HTTP semantics, contracts, DI boundaries, security/operability).
.agents/skills/fastapi-rest-api-design/REST_API_CHECKLIST.md	Provides a concrete checklist to apply during endpoint creation/review.