Add quiet mode to scan output

[rzigante]

Summary

• add --quiet / -q to raguard scan
• suppress rich banners, summaries, save messages, CI pass messages, and API-key warnings in quiet mode
• keep findings and error output visible
• document quiet mode in README and usage docs

Closes #6

Verification

• .venv312/bin/python -m pytest tests/test_reporters.py tests/test_cli_extended.py -q
• .venv312/bin/python -m pytest tests/ -q
• .venv312/bin/python -m ruff check src/raguard/cli.py src/raguard/reporters/console.py tests/test_reporters.py tests/test_cli_extended.py tests/test_cli_extra.py
• .venv312/bin/python -m py_compile src/raguard/cli.py src/raguard/reporters/console.py tests/test_reporters.py tests/test_cli_extended.py tests/test_cli_extra.py
• .venv312/bin/python -m raguard.cli scan http://localhost:8000 --quiet --api-key test-key
• .venv312/bin/python -m raguard.cli scan --help