[WIP] ✨ Set up Copilot instructions

[Copilot]

• Explore repository structure and understand the codebase
• Validate build process (yarn build - 40 seconds)
• Validate test suite (yarn test - 21 seconds, 905 tests)
• Test core functionality (encode/decode operations)
• Document existing issues (lint errors, codegen requirements)
• Measure timing for all major commands
• Create comprehensive GitHub Copilot instructions file
• Test practical development scenarios
• Validate instructions work end-to-end with fresh repository clone
• Test benchmark functionality and performance claims
• Validate all commands and workflows described in instructions

Created comprehensive GitHub Copilot instructions in .github/copilot-instructions.md that provide:

Key Features:

• Step-by-step setup and build instructions with exact timing expectations
• Clear warnings about long-running operations with "NEVER CANCEL" guidance
• Detailed project structure navigation
• Practical validation scenarios with working code examples
• Documentation of known issues and limitations
• Performance testing guidance
• Model-specific import examples

Validation Completed:

• All commands tested and work as documented
• Build times: ~40 seconds (validated)
• Test suite: ~21 seconds with 905 tests (validated)
• Benchmark suite: ~60 seconds with performance comparisons (validated)
• Functional examples: encode/decode and model-specific imports (validated)
• Fresh repository workflow tested from clone to full functionality

Critical Timing Information:

• yarn install: 12-70 seconds (varies by cache)
• yarn build: ~40 seconds
• yarn test: ~21 seconds
• yarn format: ~2 seconds
• Benchmark run: ~60 seconds

The instructions include explicit timeout recommendations and "NEVER CANCEL" warnings for all operations over 2 minutes, ensuring GitHub Copilot agents won't prematurely terminate builds or tests.

Fixes #76.

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