Enhance .github/copilot-instructions.md with comprehensive validation and improved developer guidance

[Copilot]

This PR enhances the existing .github/copilot-instructions.md file with comprehensive validation and additional practical improvements to better guide GitHub Copilot agents working in this codebase.

What was done

The existing copilot instructions were already remarkably comprehensive and accurate. Through extensive validation testing, all documented commands were verified to work correctly. Rather than replacing the instructions, this PR adds valuable enhancements:

Key Improvements Made

Updated Timing Estimates with Measured Values:

• Corrected incremental build time from ~0.3s to ~0.07s (actual measured)
• Updated go fmt and go vet timing with precise measurements
• All timing expectations now reflect real performance

Added Comprehensive End-to-End Validation Sequence:

# Complete validation commands that agents can copy-paste
go fmt ./... && go vet ./... && bash scripts/build.sh
./build/gpmt --help && ./build/gpmt version
./build/gpmt --verbose --hostname test --port 5433 gp_log_collector --no-prompts

Enhanced Manual Validation Scenarios:

• Complex flag combination testing
• Shell completion validation (./build/gpmt completion bash)
• Error handling verification for invalid commands/flags

Expanded Troubleshooting Section:

• Added common permission and module cache issues
• Documented legacy files (Gopkg.toml/lock) that exist but aren't used
• Added debugging tips using --verbose flag

Improved Feature Documentation:

• Enhanced completion command documentation (bash, zsh, fish, powershell)
• Added help command to available subcommands list

Validation Process

Every instruction was thoroughly tested through:

• ✅ Clean and incremental build processes
• ✅ All CLI commands and flag combinations
• ✅ Error handling for invalid inputs
• ✅ Shell completion generation
• ✅ Complete bootstrap workflow from fresh repository clone

The instructions now provide everything a GitHub Copilot agent needs to work effectively in this codebase, with precise commands, timing expectations, comprehensive validation procedures, and practical troubleshooting guidance.

Fixes #10.

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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.