Replace COG_TEST_FAST with t.Short() for test skipping
Replace custom COG_TEST_FAST environment variable with Go's standard
testing.Short() mechanism across all tests.
Changes:
- Use t.Short() in concurrent_test.go instead of COG_TEST_FAST check
- Remove custom [fast] condition from suite_test.go (testscript has
built-in [short] support)
- Replace [fast] with [short] in all .txtar test files
- Update documentation in CONTRIBUTING.md, README.md, and design docs
This aligns with Go testing conventions where -short flag is the
standard way to skip slow tests.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
