Add setup.ps1 for native Windows/PowerShell support

[theDakshJaitly]

Summary

setup.sh is bash-only. Windows users currently need WSL or Git Bash to run setup (see #10). A native PowerShell script would make onboarding seamless on Windows.

Full docs: https://launchx.page/mex/docs

What needs to happen

Port setup.sh to setup.ps1 with the same flow:

1. Build the CLI (npm install && npm run build)
2. Detect project state (existing codebase / fresh project / partial scaffold)
3. Tool config selection (copy from .tool-configs/ to project root)
4. Pre-analyze codebase via mex init --json (if CLI built successfully)
5. Print or run the population prompt

Scope

• Same user-facing behavior as setup.sh
• Use PowerShell-native equivalents (Write-Host with colors, Test-Path, etc.)
• The banner/spinner are nice-to-have — core flow is the priority
• Must handle the same edge cases: running from wrong directory, missing Node.js, existing files (overwrite prompt)

Reference

• setup.sh — the source of truth (~275 lines of actual logic, rest is the prompt string)
• sync.sh and update.sh — these could be ported later, not part of this issue

Acceptance criteria

• setup.ps1 runs on PowerShell 5.1+ and PowerShell Core 7+
• Same 6-step flow as setup.sh
• --dry-run and --help flags work
• Tested on Windows 10/11 without WSL

[theDakshJaitly]

Update: As of v0.2.0, npx promexeus setup replaces the bash script for most users. This is cross-platform TypeScript — works on Windows natively without PowerShell porting.

This issue is still relevant for users who prefer the git clone install path, but the priority is lower now since npx is the recommended flow.