Add GitHub Copilot repository instructions with development workflow guidance

[Copilot]

This PR adds repository-specific instructions for GitHub Copilot to help it provide better assistance when working with the OpenTelemetry Rust codebase.

The new .github/copilot-instructions.md file includes:

Development Workflow

• Individual crate development: Use focused cargo build, cargo test, cargo clippy commands within specific crates for efficient development
• Final validation: Position workspace scripts (./scripts/precommit.sh, ./scripts/lint.sh, ./scripts/test.sh) as final checks before committing rather than per-change requirements
• Regular tooling: Use clippy, rustfmt, and cargo check regularly as specified in the original issue

Comprehensive Development Guidance

• Workspace-aware commands for this multi-crate repository structure
• Feature flag patterns including signals (traces, metrics, logs) and experimental features (otel_unstable)
• Code standards including MSRV (1.75), error handling patterns, and conventional commit format
• Build requirements including protoc dependency for OTLP components
• Testing and validation practices across the entire workspace
• OpenTelemetry-specific patterns and specification compliance guidelines
• Configuration priority: compile-time config over environment variables

The instructions follow GitHub's documentation for repository instructions and provide context-specific guidance that will help Copilot understand the unique development practices, tooling, and patterns used in this OpenTelemetry implementation.

Fixes #3117.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.5%. Comparing base (c7e47de) to head (787d9f6).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3118     +/-   ##
=======================================
- Coverage   80.5%   80.5%   -0.1%     
=======================================
  Files        126     126             
  Lines      22218   22218             
=======================================
- Hits       17890   17889      -1     
- Misses      4328    4329      +1     

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[cijothomas]

@copilot code should have higher priority than env variables.

[Copilot]

Fixed the configuration priority order to prioritize compile-time config over environment variables. Changes committed in commit 8b5a9e2.

[cijothomas]

@copilot perhaps its easier to just do cargo build/test from the individual crate when working on features for that crate alone. and scripts are anyway run in CI checks, but they can used for a final check, not something to do with each change etc.

[Copilot]

Updated the development workflow guidance to emphasize individual crate commands for focused development and position the scripts as final validation tools. Changes committed in dcdeb96.