Add AI Instructions file(s)

Analyze this workspace and create or update .github/copilot-instructions.md. The file guides future AI coding agents here.

Add:

Core commands, especially build, lint, test (incl. single-test run), docs, migrations, etc.
High-level architecture, including major packages, services, data stores, external APIs, etc.
Repo-specific style rules, including formatting, imports, typing, naming, error handling, etc.
Relevant agent rules detected in .cursor/**, .cursorrules, AGENTS.md, AGENT.md, CLAUDE.md, .windsurfrules, existing Copilot file, etc.
Summarize important parts of README or other docs instead of copying them.
Guidelines (read more at [https://aka.ms/vscode-instructions-docs](vscode-file://vscode-app/Applications/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html)):

If .github/copilot-instructions.md exists, patch/merge. Never overwrite blindly.
Be concise; skip boilerplate, generic advice, or exhaustive file listings.
Use Markdown headings + bullets; keep prose minimal and non-repetitive.
Cite only facts found in the repo (don't invent information).