feat: add GitHub Copilot instructions

[Copilot]

Configures repository-specific guidance for GitHub Copilot as recommended in best practices.

Changes

• Created .github/copilot-instructions.md with:
  • Development setup (build, test, lint commands)
  • Code style distilled from STYLE.md (naming, imports, error handling, formatting)
  • Testing conventions (gopkg.in/check.v1 framework, naming patterns)
  • Project-specific patterns (concurrency, regex compilation, string operations)
  • Commit message format (conventional commits)
  • Project structure and documentation references

Key conventions captured:

• Error messages start with "cannot" and are lowercase
• Log messages start with capital letter
• Three-group import organization (stdlib, third-party, pebble)
• MixedCaps/mixedCaps naming, not underscores
• Compile regex at package level with regexp.MustCompile()

Original prompt

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This section details on the original issue you should resolve

<issue_title>AI: set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

This will be the first stepping stone towards AGENTS.md or similar.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes AI: set up Copilot instructions #739

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