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

scripts/gen_synth_pd.py

@@ -95,7 +95,10 @@ def gen():
     connect(c, panscale, 0, dac, 1)
 
     # ── Write ──
-    lines = ["#N canvas 0 0 900 550 10;"]
+    lines = [
+        "#N canvas 0 0 900 550 10;",
+        "# DO NOT EDIT — generated by scripts/gen_synth_pd.py;",
+    ]
     for x, y, t, *args in o:
         joined = " ".join(str(a) for a in args)
         lines.append(f"#X obj {x} {y} {joined};")