Add Copilot instructions and setup workflow

[Copilot]

Extracts Copilot setup files from PR #326 into a standalone PR with fixes.

Changes

.github/copilot-instructions.md

• Repository structure, coding conventions, control patterns
• Build instructions for multi-platform MAUI development
• Common issues and troubleshooting

.github/workflows/copilot-setup.yml

• Triggers: push (main branch) + workflow_dispatch
• Path filters: only runs on changes to source/tests/copilot files
• Multi-platform matrix: Ubuntu (Android), Windows, macOS
• Verifies build and runs tests

Fixes from original PR #326

• Added missing push trigger
• Normalized dotnet-version format across jobs
• Removed --no-restore from test step (restore runs in prior step)

Original prompt

Can you extract th. Copilot instructions and setup workflows in #326 and make a standalone PR. Identiftly issues with thos files and fix. We need to have a workflow that sets things up for Copilot and allows copilot to just start working. Make sure the workflow is also dispatch able. Also, make it run on push.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.