Add PHP CLI working directory support

[benitoalba]

Motivation for the change, related issues

The php command currently requires absolute VFS paths for scripts and related configuration, even when a plugin or theme has already been auto-mounted. This makes common commands such as PHPUnit unnecessarily verbose.

Fixes #3803.

Implementation details

• Add a PHP-command-only --cwd=<vfs-path> option and expose it through RunCLIArgs.
• Use an explicit CWD when provided; otherwise infer the sole auto-mounted VFS path only when no manual mounts are configured.
• Pass the CWD together with the PHP CLI invocation so the selected pooled worker changes directory before execution.
• Preserve the existing default working directory for zero, manual, and ambiguous multi-mount configurations.
• Report invalid working directories with a descriptive filesystem error.
• Document php, --cwd, and relative PHPUnit command/configuration paths.

Testing Instructions (or ideally a Blueprint)

npm exec nx -- run playground-cli:test-playground-cli --testFiles=packages/playground/cli/tests/run-cli.spec.ts
npm exec nx -- run playground-cli:test-playground-cli
npm exec nx -- run playground-cli:lint
npm exec nx -- run playground-cli:typecheck
npm exec nx -- run playground-cli:build
npm exec nx -- run php-wasm-universal:lint
npm exec nx -- run php-wasm-universal:typecheck
npm exec prettier -- --check packages/playground/cli/src/run-cli.ts packages/php-wasm/universal/src/lib/php-worker.ts packages/php-wasm/universal/src/lib/php.ts packages/playground/cli/tests/run-cli.spec.ts packages/playground/cli/README.md packages/docs/site/docs/developers/05-local-development/04-wp-playground-cli.md packages/docs/site/docs/main/guides/phpunit-testing.md
git diff --check

The full Playground CLI suite passes with 190 tests passed and 6 platform-specific tests skipped.