- Replace build stub with full phase execution (prepare/build/check/install)
via /bin/sh with variable interpolation, config file generation, and
env handling (hard-set, soft/?=, append/+=)
- Add += append operator for environment variables across DSL, parser,
system config parser, build backend, and formatter
- Add STYLEGUIDE.md documenting all codebase conventions
- Replace EnvEntry bool soft with EnvMode enum (Set/Soft/Append)
- Add Plus token type to lexer for += parsing
