feat(constants): add barrel file and fix test imports

Created barrel file to consolidate constants from /src/constants/ directory
and fixed import paths in test files.

Changes:
- Added src/constants.mts barrel file with 249 named exports
- Provides default export with all constants from ENV plus additional constants
- Fixed src/flags.test.mts import path (../constants → ./constants)
- Fixed src/constants.test.mts expectation for shadowBinPath

Impact:
- Centralized constants access through single entry point
- All 12 constants tests now passing
- flags.test.mts import error resolved

Author: jdalton