Add a `code-review` agent skill for GitHub Copilot code review

📝 Scenario

As a maintainer reviewing pull requests with GitHub Copilot code review, I need Copilot's automated reviews to apply the FinOps Toolkit's coding standards (Bicep style, PowerShell conventions, KQL operator rules, KQL join kind= rules, content/style guide, etc.) in order to get more relevant, higher-signal feedback on PRs without repeating that context manually in every review.

💎 Solution

GitHub Copilot code review can use agent skills and MCP servers configured in a repository when they're relevant to the code being reviewed. Skills live under .github/skills, and Copilot is more likely to use them for review when the skill directory has a review-focused name/description, such as code-review.

Reference: Using GitHub Copilot code review – MCP servers and agent skills

Add a .github/skills/code-review agent skill to the toolkit that encodes our existing review standards so Copilot's PR reviews are grounded in them, for example:

• The content/coding standards in docs-wiki/Coding-guidelines.md (sentence casing, Microsoft style guide, Bicep style guide, PowerShell approved verbs/best practices).
• KQL-specific rules: no tolower()/toupper() in comparisons, explicit kind= on every | join, lookup vs join guidance, leftanti for exclusions, and the ARG exception.
• Any other review-worthy conventions already enforced by lint tests (e.g. HubsKqlOperators.Tests.ps1, KqlJoinKinds.Tests.ps1) that a reviewer should flag on new code even before CI runs.

See Adding agent skills for GitHub Copilot for the skill file format/setup.

📋 Tasks

### Required tasks
- [ ] Create `.github/skills/code-review/` with a `SKILL.md` (or equivalent) describing review scope and instructions
- [ ] Encode Bicep, PowerShell, KQL, and content/style standards from `docs-wiki/Coding-guidelines.md` into the skill
- [ ] Verify Copilot code review picks up and applies the skill on a test PR
- [ ] Document the new skill (e.g. in `docs-wiki/Coding-guidelines.md` or contributor docs) so contributors know it exists

### Stretch goals
- [ ] Add path-specific `.github/instructions/**/*.instructions.md` files to complement the skill for language-specific review guidance
- [ ] Evaluate enabling automatic Copilot code review on PRs once the skill is validated

ℹ️ Additional context

This repo already has .github/copilot-instructions.md and AGENTS.md for general repo context, but no .github/skills directory yet. A dedicated code-review skill is additive and specifically targeted at review tasks, per GitHub's guidance that skills with review-focused names/descriptions are more likely to be used during Copilot code review.

🙋‍♀️ Ask for the community

We could use your help:

1. Please vote this issue up (👍) to prioritize it.
2. Leave comments if you have suggestions for what review standards should be included.

Start by reading docs-wiki/Coding-guidelines.md, .github/copilot-instructions.md, and AGENTS.md, then review HubsKqlOperators.Tests.ps1 and KqlJoinKinds.Tests.ps1 for enforced review rules. Add and document .github/skills/code-review/SKILL.md, then verify the skill on a test PR; done means Copilot applies the stated Bicep, PowerShell, KQL, and style guidance.