Add build-firmware skill: native ESP-IDF build, no Docker needed

CI builds firmware inside the espressif/idf Docker image, but this
sandbox can't run containers at all -- it strips cap_sys_admin (and
blocks unshare) from the capability set even for root, which container
image-layer extraction and namespace setup both need. Confirmed by
hand: docker.io installs and dockerd starts fine, but even a bare
`docker run hello-world` fails to extract its own layer.

Works around it by installing ESP-IDF natively instead (git clone +
its own install.sh, scoped to just this project's esp32c6 target) --
the same way a developer would set it up on their own machine, needing
nothing this sandbox disallows. Verified end-to-end: both board
variants (devkit, xiao) build clean from a fresh checkout via the
packaged setup.sh/build.sh.

.claude/skills/build-firmware/build.sh

@@ -0,0 +1,63 @@
+#!/usr/bin/env bash
+# Builds (or flashes/monitors, if a serial port is actually attached)
+# the espresso_frame firmware for one board variant, via the project's
+# own firmware/build_for_board.sh -- this script just sources the
+# ESP-IDF environment first and auto-runs `set-target esp32c6` on a
+# board's very first build (a fresh clone has no generated sdkconfig
+# yet, same reasoning as CI's own build steps -- see firmware/README.md's
+# "Building for the Seeed XIAO ESP32-C6" section).
+#
+# Usage:
+#   build.sh                        # build devkit (default)
+#   build.sh devkit
+#   build.sh xiao
+#   build.sh both                   # build both board variants
+#   build.sh xiao flash -p /dev/ttyUSB0   # only meaningful with real hardware attached
+set -euo pipefail
+
+script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+firmware_dir="$(git -C "$script_dir" rev-parse --show-toplevel)/firmware"
+
+IDF_DIR="$HOME/.espressif-idf/esp-idf"
+if [ ! -f "$IDF_DIR/export.sh" ]; then
+  echo "ESP-IDF not found at $IDF_DIR -- run setup.sh first" >&2
+  exit 1
+fi
+# export.sh is chatty and assumes an interactive shell prompt in spots;
+# redirect its own stdout, not ours, so build.sh's actual output (and a
+# real failure's stderr) stays visible.
+source "$IDF_DIR/export.sh" > /dev/null
+
+cd "$firmware_dir"
+
+build_one() {
+  local board="$1"
+  shift
+  local sdkconfig
+  case "$board" in
+    devkit) sdkconfig="sdkconfig" ;;
+    xiao) sdkconfig="sdkconfig.xiao_local" ;;
+    *) echo "Unknown board '$board' -- expected 'devkit' or 'xiao'" >&2; exit 1 ;;
+  esac
+
+  if [ ! -f "$sdkconfig" ]; then
+    echo "==> $board: no generated sdkconfig yet, setting target esp32c6"
+    ./build_for_board.sh "$board" set-target esp32c6
+  fi
+
+  local args=("$@")
+  if [ ${#args[@]} -eq 0 ]; then
+    args=(build)
+  fi
+  ./build_for_board.sh "$board" "${args[@]}"
+}
+
+board="${1:-devkit}"
+shift || true
+
+if [ "$board" = "both" ]; then
+  build_one devkit "$@"
+  build_one xiao "$@"
+else
+  build_one "$board" "$@"
+fi