docs: add Copilot instructions for code review guidance

Instructs code review agents to:
- Focus on build scripts, workflows, and psdocs-azure package
- Treat packages/psdocs and packages/vscode-extension as vendored
  (git subtree imports) - skip unless critical security issue

Author: vicperdana

.github/copilot-instructions.md

@@ -0,0 +1,43 @@
+# Copilot Instructions for PSDocs.Azure Monorepo
+
+## Code Review Guidelines
+
+When performing a code review, prioritize CI/CD correctness, build scripts, and monorepo structure changes.
+
+### Focus review comments on:
+- `build.ps1` - Root build orchestrator
+- `build/common.ps1` - Shared build utilities
+- `.github/workflows/**` - CI/CD workflows
+- `.github/skills/**` - Copilot skills
+- `packages/psdocs-azure/**` - Core PSDocs.Azure module
+- `MONOREPO_MIGRATION.md` - Migration documentation
+
+### Treat these paths as imported/vendored (git subtree). Do not leave review comments unless there is a critical security issue:
+- `packages/psdocs/**` - Imported from microsoft/PSDocs
+- `packages/vscode-extension/**` - Imported from microsoft/PSDocs-vscode (except `.github/workflows/` changes)
+
+## Build System
+
+This is a PowerShell-based monorepo using InvokeBuild:
+
+```powershell
+# Build all packages
+./build.ps1 -Build
+
+# Build specific package
+./build.ps1 -Package psdocs-azure -Build -Test
+```
+
+## Versioning
+
+Each component uses independent versioning with prefixed tags:
+- PSDocs: `psdocs-v{version}`
+- PSDocs.Azure: `psdocs-azure-v{version}`
+- VS Code Extension: `vscode-v{version}`
+
+## CI/CD
+
+Path-based filtering triggers builds only for changed packages:
+- `packages/psdocs/**` → PSDocs build
+- `packages/psdocs-azure/**` → PSDocs.Azure build
+- `packages/vscode-extension/**` → VS Code extension build