Migrates CI testing from CircleCI to GitHub Actions while preserving all test coverage.

Changes

• New workflow: .github/workflows/test.yml

  • Matrix testing across Python 3.9, 3.11, 3.13
  • Runs pytest with JUnit XML output
  • Executes Color TextModeTrees validation
  • Runs external_tests.sh integration tests
  • Uploads test results as artifacts per Python version

• Build artifacts: Updated .gitignore to exclude build/, *.egg-info/, test-results/

Workflow triggers

on:
  push:
    branches: [ "master", "main" ]
  pull_request:
    branches: [ "master", "main" ]

The existing .circleci/config.yml remains in place for gradual migration.

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