Skey: a FIDO2 security key on the Raspberry Pi Pico

Firmware for the Pico / Pico 2 that acts as a USB FIDO2 authenticator:
CTAP2 and U2F, passkeys, BOOTSEL as the touch button, and clientPIN with
PIN/UV auth protocols 1 and 2. A host simulator runs the same core so the
python-fido2 test suite exercises it without hardware.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

firmware/button.h

@@ -0,0 +1,5 @@
+#pragma once
+
+// Samples the touch button. Call from the main loop so presses made between
+// requests are seen.
+void button_poll(void);