Add --device flag to select microphone by name

[turqoisehex]

Summary

hushtype auto-detects NVIDIA Broadcast virtual mic via find_input_device() (line ~107), but if Broadcast is not installed, it falls back to the system default. There is no way to specify which microphone to use from the command line.

A --device flag would let users pick a specific microphone by name substring (e.g., --device "Blue Yeti"), using the existing find_input_device() function.

Acceptance Criteria

• Add --device NAME argument to argparse (parse_args(), line ~486)
• If --device is provided, use find_input_device(NAME) to find the matching device
• If no match found, print available devices and exit with a clear error
• If --device is not provided, keep current behavior (auto-detect Broadcast, then default)
• Update README Usage section and --help output

Relevant Code

• find_input_device(): hushtype.py line ~107
• NVIDIA Broadcast detection: hushtype.py in create_recorder()
• argparse setup: hushtype.py parse_args() line ~486

Notes

The find_input_device() helper already exists and does substring matching. This issue is mostly wiring it to a CLI flag and adding a "list devices" fallback on no match.