refactor: rename sketch to sketch.ino, add flash-sketch recipe

- Permanent rename: led-matrix-bridge.ino → sketch.ino (Arduino CLI convention)
- Add just flash-sketch recipe for compiling/flashing MCU via board's arduino-cli
- Run via post-merge hooks for auto-deploy

justfile

@@ -338,6 +338,12 @@ kconfig:
     @ssh {{SSH_OPTS}} "{{BOARD_USER}}@{{BOARD_HOST}}" "\
         zgrep -E 'CONFIG_USB_(GADGET|CONFIGFS|F_UAC1|F_MIDI|DWC3|F_ACC|F_SERIAL)' /proc/config.gz 2>/dev/null | awk -F= '{printf \"  %-45s %s\\n\", \$1, \$2}'"
 
+# Compile and flash the MCU sketch (LED matrix bridge) via SWD
+flash-sketch:
+    @ssh {{SSH_OPTS}} "{{BOARD_USER}}@{{BOARD_HOST}}" "\
+        cd {{REPO_DIR}} && \
+        arduino-cli compile --upload --fqbn arduino:zephyr:unoq sketch/"
+
 # Validate all .pd files using Conftest/OPA Rego policies
 validate:
     python3 pd-validator/validate.py pd/*.pd