Add comprehensive .github/copilot-instructions.md for GitHub Copilot coding agent

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with detailed instructions for working effectively in the Protols codebase.

What's Included

The instructions file provides complete guidance for:

Build & Development Workflow:

• Step-by-step setup with exact commands and validated timing expectations
• Rust toolchain usage (cargo build ~1 min, cargo test ~6 sec, cargo clippy ~15 sec)
• External dependency setup (protoc, clang-format) with verification commands
• Critical timeout warnings ("NEVER CANCEL" with 90+ minute timeouts for builds)

Manual Validation Scenarios:

• 4 comprehensive test scenarios covering build validation, dependency verification, LSP functionality testing, and sample file validation
• Specific commands for testing core LSP features (hover, workspace operations, go-to-definition)
• Integration testing with Protocol Buffer files using protoc and clang-format

Project Navigation & Structure:

• Detailed source code organization (src/lsp.rs, src/parser/, src/workspace/, etc.)
• Key files and their purposes (1400+ lines across 22 test modules)
• Sample files for testing (sample/simple.proto, test input files)

Development Best Practices:

• Common debugging workflows and log file locations
• Performance expectations and timing guidelines
• CI validation requirements (cargo fmt --check, cargo clippy, full test suite)

Validation

Every single command in the instructions has been tested and validated to work correctly:

• ✅ All build commands execute successfully with measured timing
• ✅ Full test suite passes (22 tests in ~6 seconds)
• ✅ External dependencies (protoc 3.21.12, clang-format 18.1.3) verified
• ✅ LSP functionality confirmed through specific test execution
• ✅ Sample proto file processing validated

The instructions follow an imperative tone ("Run this command", "Always test") and provide copy-pasteable commands that agents can execute reliably to build, test, and validate changes to the Language Server Protocol implementation.

Fixes #92.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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[yashpathack]

This should be 0.12.9 after the recent version bump. Maybe add a template script to keep {{VERSION}} updated automatically?