Review and update export-ignore rules in .gitattributes

[j13k]

Review the .gitattributes file to ensure export-ignore rules are appropriate and complete for archive exports (e.g., composer archive, GitHub release archives).

Current State

The repository has a .gitattributes file with export-ignore rules that control which files are excluded from distribution archives.

Review Needed

• Verify all development/tooling files are properly excluded
• Check if new files need to be added to export-ignore:
  • CONTRIBUTING.md (should this be included in archives?)
  • captainhook.json (development only)
  • CLAUDE.md (development documentation)
  • .github/ directory
  • CI/CD configuration files
  • Development scripts or tools
• Ensure test files and fixtures are excluded
• Verify documentation files that should be included are NOT excluded

Goals

• Minimise archive size by excluding unnecessary files
• Ensure only production-relevant files are distributed
• Follow best practices for PHP package distribution
• Document any decisions about edge cases (e.g., should CONTRIBUTING.md be in archives?)

Testing

After changes, verify with:

git archive HEAD | tar -t