This PR creates comprehensive GitHub Copilot instructions that tell coding agents exactly how to work effectively in the WealthsimpleDownloader codebase. The instructions provide validated commands, exact timeouts, and complete workflows for building, testing, and validating changes.

Key Features

Validated Commands with Exact Timeouts:

• swift build - 5 seconds actual runtime, 60+ second timeout recommended
• swift test --enable-code-coverage -Xswiftc -warnings-as-errors - 4 seconds actual, 30+ second timeout
• SwiftLint installation and execution via curl download (GitHub Actions compatible)

Complete Validation Workflow:

• Build → Test with coverage and strict warnings → Lint with strict rules → Manual library validation
• Includes "NEVER CANCEL" warnings with specific time expectations for all long-running operations
• Manual library validation pattern that ensures API compatibility after changes

Swift Package Manager Focus:

• Pure Swift library (no runnable applications)
• Documents the Swift 6.1.2+ requirement and multi-platform support
• Explains the library-only nature and API-focused validation approach

CI Integration:

• Commands match exactly what runs in .github/workflows/ci.yml
• SwiftLint 0.59.1 installation mirrors the CI pipeline
• Coverage enforcement and strict warning compilation included

Comprehensive Reference:

• Repository structure with actual directory listings
• Key Package.swift content for quick reference
• Project details including platforms, dependencies, and documentation links

The instructions start with a firm directive to "always reference these instructions first and fallback to search or bash commands only when you encounter unexpected information" to ensure agents follow the validated workflow.

All commands have been exhaustively tested from scratch to ensure they work exactly as documented, including timeout validation and complete end-to-end scenarios.

Fixes #247.

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.