Add PowerShell setup-pattern scanning

[Pavithran-R-A]

Problem

SetupSentry currently only detects dangerous patterns in bash/sh shell commands. Windows-based projects and coding agents using PowerShell may contain dangerous setup patterns that go undetected.

Proposed Scope

Add rule(s) that detect dangerous PowerShell setup patterns, such as:

• Invoke-WebRequest piped to Invoke-Expression
• DownloadString piped to execution
• IEX / Invoke-Expression with remote content
• Start-Process with untrusted URLs
• Certificate bypass ([Net.ServicePointManager]::SecurityProtocol)

Files/Components

• New rule file(s) in src/rules/
• Tests in tests/rules.test.ts
• Update src/discovery.ts to discover .ps1 files if warranted
• Update README with PowerShell coverage notes

Acceptance Criteria

• At least one new rule detecting PowerShell-specific dangerous patterns
• Positive and negative test cases
• Rule registered in src/rules/index.ts
• README updated to reflect PowerShell coverage (or scope limitation explained)
• All existing tests still pass