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

justfile

@@ -341,6 +341,10 @@ kconfig:
 validate:
     python3 pd-validator/validate.py pd/*.pd
 
+# CI: run full validation suite (same as validate but used by Gitea Actions)
+ci:
+    python3 pd-validator/validate.py pd/*.pd
+
 # Install udev rules for hotplug role changes
 install-udev-rules:
     @ssh {{SSH_OPTS}} "{{BOARD_USER}}@{{BOARD_HOST}}" "\