feat: implement --quiet flag for clean, scriptable output

Add comprehensive quiet mode support that suppresses all UI elements
while preserving AI responses and tool outputs for automation use cases.

Changes:
- Add --quiet/-q flag that implies --no-interactive
- Suppress welcome banner, tips, shortcuts, and separators
- Suppress model information and trust messages
- Suppress tool execution UI (headers, completion messages, descriptions)
- Suppress response prefix ('> ')
- Suppress MCP/agent loading messages
- Suppress tool UI feedback messages

The --quiet flag enables clean output suitable for:
- Scripting and automation
- CI/CD pipelines
- Programmatic consumption
- Integration with other tools

Only AI responses and actual tool outputs are displayed in quiet mode.

Author: Olivier Mansour