[Phase 1a] Implement custom PSHost for prompt interception

[usepowershell]

Part of #98

Implement custom PSHost/PSHostRawUserInterface to intercept prompts.

Subtasks

• Design custom PSHost class
• Implement ReadLine() interception
• Implement ReadLineAsSecureString() interception
• Implement PromptForChoice() interception
• Thread safety for concurrent access
• Unit tests
• Integration tests

Effort: 2-3 weeks