Run isolated shell commands against the PHP VFS

[chubes4]

Fixes #2304.

What changed

• Adds a Node-only shell executor backed exclusively by the active PHP virtual filesystem.
• Supports compound shell syntax, redirects, pipelines, command substitution, executable scripts, stdin/status propagation, and deployment commands used by CodeDeployments.
• Adds VFS implementations for unzip, rsync, mktemp, GNU-style ISO dates, and the jq bindings needed by the command logger.
• Routes shell filesystem operations to the originating PHP instance so files created through proc_open() are immediately visible to the parent PHP process.
• Retains an acquired PHP worker as the concurrency lease for pipe I/O and as the execution target for actual PHP subprocesses.
• Keeps the universal/browser package free of Node-specific shell dependencies.

Verification

• Asyncify focused shell suite: 11/11 passed.
• JSPI focused shell suite: 11/11 passed.
• CLI parent-VFS regression passed.
• php-wasm-node, php-wasm-universal, and Playground CLI typechecks passed.
• PHP-WASM Node, universal, Playground CLI, and Playground WordPress lint passed.

Downstream status

The source behavior is verified, including parent-visible mutations. The combined WPCOM parity rerun is currently blocked before PHPUnit by the separate external-extension loader ABI stack (resolved is not a function), so this remains draft while that package composition is restored.

AI assistance

OpenCode using openai/gpt-5.6-sol implemented the shell/VFS integration, added the focused regressions, and ran the verification listed above. Chris Huber remains responsible for the submitted change.

AI assistance: OpenAI gpt-5.6-sol via OpenCode was used to review and implement the follow-up fixes and tests.