Add licensing: CC BY-NC-SA 4.0 hardware, GPL-2.0 firmware

Hardware and docs get CC BY-NC-SA 4.0 - build it, change it, share changes,
no commercial products. Firmware has to be GPL-2.0 because it derives from
QMK, and the GPL forbids adding a NonCommercial restriction, so that half
permits commercial use whether we like it or not.

LICENSE.md records both, plus the two honest caveats: NC means this is not
open source by the OSI or OSHWA definitions, and NonCommercial is a vague
term whose enforcement falls on the copyright holder.

Co-Authored-By: Claude Opus 5 <[email protected]>

firmware/README.md

@@ -0,0 +1,9 @@
+# Firmware
+
+Nothing here yet. See the Firmware notes in the top-level README for what's
+already known — the `SPLIT_USB_DETECT` trap, the Num Lock resolve-at-press
+design, and the encoder config.
+
+**Licence: GPL-2.0-or-later.** Anything in this directory derives from QMK and
+inherits its licence. Unlike the hardware, it carries no NonCommercial
+restriction — the GPL does not permit adding one.