Add parallel test execution for local development

[mroderick]

Summary

This PR introduces parallel test execution for local development, significantly reducing test runtime.

Changes

1. Added make test target to Makefile

Runs tests in parallel using 3 processes (optimal for this codebase):

bundle exec parallel_rspec spec/ -n 3

2. Made SimpleCov conditional on COVERAGE environment variable

• SimpleCov is now only enabled when COVERAGE=true
• Speeds up local test runs by avoiding coverage overhead
• Prevents coverage file conflicts when running parallel tests locally
• CI already has COVERAGE=true set, so coverage reporting continues to work

3. Added parallel test support for coverage merging

When TEST_ENV_NUMBER is set (by parallel_tests gem) or CI_NODE_INDEX (GitHub Actions), coverage results are properly namespaced for merging.

4. Updated AGENTS.md documentation

• Recommends make test for parallel test execution
• Notes that SimpleCov only runs when COVERAGE=true
• Fixed outdated bin/drspec reference

Performance Results

Command	Time	Improvement
bundle exec rspec	~3m2s (182s)	baseline
make test	~1m39s (99s)	~45% faster

Usage

# Run all tests in parallel (recommended)
make test

# Or run directly with parallel_rspec
bundle exec parallel_rspec spec/ -n 3

# Run single test (no parallelization needed)
bundle exec rspec spec/path/to/file_spec.rb:42

# Run with coverage (local)
COVERAGE=true bundle exec rspec

Notes

• The -n 3 (3 processes) was chosen as optimal for this codebase. Higher values don't provide significant additional speedup due to database contention.
• CI already uses parallel test execution across 6 matrix jobs, so no changes needed there.