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

system/pd-synth-auto.service

@@ -1,5 +1,6 @@
 [Unit]
 Description=Pure Data Synth (auto-detect: gadget or host)
+ConditionPathExists=/home/arduino/uno-q-audio-synth/pd/synth.pd
 Documentation=https://github.com/arduino/uno-q-audio-synth
 After=usb-role-detect.service
 Wants=usb-role-detect.service

system/pd-synth-i2s.service

@@ -1,5 +1,6 @@
 [Unit]
 Description=Pure Data I2S DAC Synth (standalone)
+ConditionPathExists=/home/arduino/uno-q-audio-synth/pd/synth.pd
 After=network.target sound.target
 Wants=usb-gadget-audio.service
 

system/pd-synth.service

@@ -1,5 +1,6 @@
 [Unit]
 Description=Pure Data USB Audio Synth (Gadget Mode)
+ConditionPathExists=/home/arduino/uno-q-audio-synth/pd/synth.pd
 After=usb-gadget-audio.service midi-bridge.service
 Wants=usb-gadget-audio.service midi-bridge.service