Add provider auto-installation prompts

[shakecodeslikecray]

Description

When no LLM provider is detected, offer to install one for the user.

Tasks

• Detect OS and package manager (npm, pip, homebrew)
• Show installation options with commands
• Optionally run installation command with user consent
• Verify installation succeeded

Acceptance Criteria

• User is prompted to install a provider when none detected
• Installation commands are correct for the OS
• User can choose which provider to install