feat: add skills, version pinning, CI/CD, and generated-file headers

- Create skills: rego-pd-validator, usb-gadget-configfs, led-matrix-msgpack-rpc
- Update pd-audio-synthesis skill with Uno Q specifics
- Add Pd version check (min 0.54.0) to start-synth.sh
- Add DO NOT EDIT headers to generated pd patches
- Add Gitea Actions CI workflow for patch validation
- Add ConditionPathExists guards to legacy systemd services
- Add 'just ci' recipe for CI

pd/mpk-mini-sim.pd

@@ -1,4 +1,5 @@
 #N canvas 0 0 950 490 10;
+# DO NOT EDIT — generated by scripts/gen_mpk_sim.py;
 #X obj 10 10 loadbang ;
 #X msg 10 30 ;;
 #X msg 10 50 connect 192.168.9.167 8081;;