Reduce the repeated base directory lookups #633

Summary

Read the base directory once where a method asks for it several times, and reuse the existing directory helpers where a path is built by hand. No behavior change.

Why

App::getBaseDir() is self::getContext()->getBaseDir(), so each call is cheap and this is about readability, not speed. It is called directly 17 times in 6 files in src/, and the base_dir() helper (src/App/Helpers/dir.php), which wraps the same call, is used 23 times in 20 files. The repetition has two forms:

• the same call several times in one method: EnvCommand::exec() (4 times), Environment (3 times: load() and the env file path) and Config (2 times);
• paths rebuilt by hand although a helper exists: LoadHelpersStage builds .../modules/*/helpers although modules_dir() exists, and ModuleLoader and Config both build .../shared/config, for which there is no shared_dir() helper.

Scope

• In EnvCommand::exec(), Environment and Config, read the base directory into a local variable once.
• Use modules_dir() in LoadHelpersStage where the modules path is rebuilt.
• Decide whether a shared_dir() helper next to modules_dir() and logs_dir() is worth a new public helper; if not, leave the two shared/config paths as they are.

Acceptance Criteria

• no method asks for the base directory more than once to build its paths
• no behavior change: the existing tests pass, and a test covers a path that moves to a helper if none does
• composer test, composer cs:check, composer phpstan and composer rector:check pass