Add configurable Codex MCP cwd

[cyppe]

Summary

• Add boost.executable_paths.current_directory config option (env: BOOST_CURRENT_DIRECTORY_EXECUTABLE_PATH) to override the MCP server cwd
• Keep the existing base_path() behavior as the default fallback
• Cover the default, config override, and env override paths in CodexTest

Why

Boost currently generates Codex MCP config with cwd = base_path(). That works when Boost and Codex both run in the same environment, but it breaks when Boost is executed inside a container and Codex runs on the host.

A concrete example is DDEV:

• php artisan boost:update runs inside the DDEV container
• Laravel base_path() resolves to a container path like /var/www/html
• Codex starts on the host, reads .codex/config.toml, and then tries to launch the MCP server with cwd = /var/www/html
• that path does not exist on the host, so MCP startup fails before the command even runs

The command path can already be overridden safely with BOOST_PHP_EXECUTABLE_PATH, but there is currently no official way to override the MCP working directory.

This uses the existing executable_paths config group rather than a new mcp.cwd key because the setting describes the working directory of an executable — consistent with how BOOST_PHP_EXECUTABLE_PATH and friends are already organized.

Safety

• Fully backward compatible because base_path() remains the fallback
• No container or platform heuristics are introduced
• Users only opt in when they need a host/container path split
• The current Codex behavior is unchanged unless BOOST_CURRENT_DIRECTORY_EXECUTABLE_PATH is set

Verification

• ./vendor/bin/pest tests/Unit/Install/Agents/CodexTest.php
• ./vendor/bin/pint --test config/boost.php src/Install/Agents/Codex.php tests/Unit/Install/Agents/CodexTest.php