fix(usb): select stream deck by serial

[jack-arturo]

Summary

• add streamdeck_list_devices to enumerate attached USB Stream Decks
• allow streamdeck_connect to select a specific deck by serial while preserving first-device default behavior
• document the multi-deck USB workflow and add mocked hardware regression tests

Fixes #44

Tests

• uv run pytest tests/test_server.py -q
• uv run pytest tests/ -q
• uv run ruff check .

Risk

• No physical multi-deck hardware test was run; coverage uses mocked python-elgato-streamdeck devices.

[Copilot]

Pull request overview

This PR adds multi-device support to the legacy USB-direct Stream Deck MCP server by allowing devices to be enumerated and selected by serial number.

Changes:

• Adds USB Stream Deck enumeration via streamdeck_list_devices.
• Extends streamdeck_connect with an optional serial argument while preserving default first-device behavior.
• Updates README guidance and adds mocked regression coverage for multi-deck selection.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
server.py	Adds device enumeration, serial selection, tool schema updates, and tool dispatch handling.
tests/test_server.py	Adds mocked tests for listing devices, serial selection, unknown serial errors, and tool schema exposure.
README.md	Documents the legacy USB multi-deck workflow.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.