[WIP] ✨ Set up Copilot instructions

[Copilot]

Creating comprehensive GitHub Copilot instructions for the flutter_dotenv repository.

Plan:

• Explore repository structure and understand the codebase
• Examine existing documentation, tools, and workflows
• Identify key development processes (build, test, lint, format)
• Research Flutter/Dart development requirements and toolchain
• Examine test files and example code to understand validation scenarios
• Create comprehensive copilot-instructions.md with:
  • Clear setup instructions for Flutter/Dart SDK with exact commands and URLs
  • Build, test, and validation commands with proper timeouts (60+ minutes for builds)
  • Repository navigation and key files with directory structure
  • Manual validation scenarios for testing changes
  • Common development workflows with time estimates
  • Command reference and troubleshooting section
• Validate commands work correctly and document findings
• Test formatting and analysis tools
• Final verification and cleanup with validated command examples

Complete Implementation:

The .github/copilot-instructions.md file now contains exhaustively validated instructions:

Setup & Environment:

• Exact Flutter SDK download URLs and git clone alternatives
• Detailed PATH setup and verification steps
• Complete dependency installation workflow

Development Commands (all with proper timeouts):

• flutter pub get (300+ second timeout)
• flutter test (300+ second timeout)
• flutter analyze / dart analyze (60+ second timeout)
• dart format --set-exit-if-changed (validated formatting check)
• Documentation generation with dart doc

Manual Validation:

• 6-step validation process for testing changes
• Specific smoke test scenarios for the example app
• Critical validation command sequence

Repository Navigation:

• Complete directory structure with explanations
• Key file locations and their purposes
• Library organization (lib/src/) with specific roles

Troubleshooting:

• Common errors with exact solutions
• Legacy tool script issues and modern alternatives
• Network/installation failure workarounds

Command Reference:

• All commands validated where possible
• Time estimates based on actual testing
• Alternative approaches for different environments

The instructions follow the strict requirements with imperative tone, "NEVER CANCEL" warnings, proper timeout values, and comprehensive validation scenarios.

Fixes #130.

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