Windows command spawning

Applies to anything that spawns a child process with arguments — spawn_streaming, socket_firewall (node-pty), and any new caller. All of them go through src/ipc/utils/windows_command.ts, which is the single source of truth so quoting/security fixes apply everywhere.

A bare command name becomes a .cmd shim

resolveWindowsExecutableName appends .cmd to any command without a . in it (npm → npm.cmd), because that's what the command really is on Windows. This means node, npx, and npm all take the cmd.exe path, not the direct-exec path — only a name with an extension (node.exe) passes through unchanged. Assuming otherwise is an easy way to write a test that asserts the wrong branch.

% and newlines cannot be passed through cmd.exe

.cmd/.bat shims can't be exec'd directly, so they're routed through cmd.exe /d /s /c with a single command string. Quoting each argument preserves shell metacharacters (&, |, <, >, ^, !, (), spaces, quotes) — which is what lets a Playwright grep regex like (adds|removes) item survive — but two things quoting cannot contain:

• %: cmd.exe expands %VAR% even inside double quotes, and %% only escapes inside a batch file, not on a command line.
• CR/LF: cmd.exe treats newlines as command separators inside double quotes.

quoteWindowsCmdArg therefore throws on both rather than silently rewriting the value into a different command. Don't "fix" a throw by stripping the characters — a caller passing model- or user-supplied text (grep patterns, filenames) needs to fail loudly, not run a mangled or injected command. If a %-bearing argument must genuinely be supported, it needs a non-cmd.exe transport, not more escaping.