fix(cli): use where on Windows in commandExists helper

[AdarshJ173]

Summary

On Windows, command -v <cmd> is a POSIX-only shell builtin that does not exist in cmd.exe. When Node's child_process.exec() runs this string on Windows, the shell immediately throws an error — causing commandExists() to return false even when the target binary (e.g. docker) is fully installed and on PATH. This makes corebrain setup (local, docker, and railway) completely unusable on Windows, even with a working Docker Desktop install.

This PR replaces the hardcoded command -v with a cross-platform check: where on win32, command -v on all other platforms — using process.platform, a value that is always available at runtime with no new imports or dependencies.

Fixes #904 — corebrain setup fails on Windows: "docker not found on PATH" when Docker is installed

What changed

packages/cli/src/utils/setup/local.ts

• Problem: commandExists used await exec(`command -v ${cmd}`) — fails silently on Windows, causing setupLocalWebapp to throw docker not found on PATH even when Docker is installed.
• Change: Replaced template literal with `${process.platform === 'win32' ? 'where' : 'command -v'} ${cmd}` — one line, zero new imports.

packages/cli/src/utils/setup/docker.ts

• Problem: Same command -v pattern copied from local.ts, affects both docker and tailscale checks in setupDockerGateway.
• Change: Same single-line cross-platform fix.

packages/cli/src/utils/setup/railway.ts

• Problem: Same command -v pattern, affects railway, brew, and npm detection in setupRailwayGateway and offerRailwayInstall.
• Change: Same single-line cross-platform fix.

Acceptance criteria

• corebrain setup no longer throws "docker not found on PATH" on Windows when Docker Desktop is installed and docker is on PATH
• corebrain gateway setup --docker correctly detects docker and tailscale on Windows
• corebrain gateway setup --railway correctly detects railway, brew, and npm on Windows
• All three setup paths continue to work unchanged on macOS and Linux

How to test

1. On a Windows machine with Docker Desktop installed, run corebrain setup
2. Expected: the CLI proceeds past the docker check without throwing
3. On macOS/Linux, run the same commands — verify no regression
4. Edge case: rename docker temporarily out of PATH on Windows — verify commandExists correctly returns false and the error message still displays
5. Edge case: run on Linux — verify command -v is still used (not where)

Notes

• No schema changes
• No new dependencies added — process.platform is a Node.js built-in
• No new imports in any file
• No server/API changes
• No breaking changes to existing consumers — commandExists is a private module-level function, not exported
• The where command has been available on Windows since XP and is present in all modern cmd.exe environments
• process.platform returns 'win32' for both 32-bit and 64-bit Windows

[harshithmullapudi]

hey @AdarshJ173 after building did it run in windows ?