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/setup.sh

@@ -0,0 +1,58 @@
+#!/usr/bin/env bash
+# One-time (idempotent) environment bootstrap for compiling the
+# espresso_frame firmware (firmware/) without Docker -- see this
+# skill's SKILL.md for why not Docker, even though that's what CI uses.
+# Re-run any time; every step is safe/fast to repeat once already done.
+set -euo pipefail
+
+IDF_ROOT="$HOME/.espressif-idf"
+IDF_DIR="$IDF_ROOT/esp-idf"
+# Matches the espressif/idf:release-v6.0 image CI's
+# firmware-build-check.yml/firmware-release-build.yml use -- keep this
+# in sync with those workflow files if the project's pinned IDF version
+# ever changes.
+IDF_BRANCH="release/v6.0"
+
+# 1. OS packages ESP-IDF's own install.sh needs (python3 + venv/pip,
+# cmake, ninja, a C toolchain for the odd host-side code generator, git
+# for the clone below, flex/bison/gperf for mbedtls/etc.'s generated
+# parsers, libusb for esptool's USB/JTAG bits even though this skill
+# doesn't flash real hardware). Installed via apt with real root --
+# unlike run-server's Chromium bootstrap, this container actually has
+# root and a working apt, so no non-root extraction dance is needed
+# here.
+PKGS="git python3 python3-venv python3-pip cmake ninja-build ccache libusb-1.0-0 wget flex bison gperf build-essential"
+missing=()
+for pkg in $PKGS; do
+  dpkg -s "$pkg" >/dev/null 2>&1 || missing+=("$pkg")
+done
+if [ ${#missing[@]} -gt 0 ]; then
+  echo "installing OS packages: ${missing[*]}"
+  apt-get update
+  DEBIAN_FRONTEND=noninteractive apt-get install -y "${missing[@]}"
+fi
+
+# 2. ESP-IDF checkout -- shallow, single branch, recursive submodules
+# also shallow (~700MB total, vs. several GB for a full clone). Only
+# clones once; re-running this script never re-clones or resets it, so
+# any local changes you made for debugging survive a re-run.
+if [ ! -d "$IDF_DIR/.git" ]; then
+  echo "cloning esp-idf $IDF_BRANCH into $IDF_DIR ..."
+  mkdir -p "$IDF_ROOT"
+  git clone --branch "$IDF_BRANCH" --depth 1 --shallow-submodules --recursive \
+    https://github.com/espressif/esp-idf.git "$IDF_DIR"
+else
+  echo "esp-idf already cloned at $IDF_DIR"
+fi
+
+# 3. Toolchain + Python virtualenv, scoped to esp32c6 only -- this
+# project's one target (see firmware/README.md's board table). Scoping
+# avoids downloading toolchains for every chip ESP-IDF supports, which
+# matters given this container's disk headroom. install.sh is already
+# idempotent on its own (checks what's present and skips it), so this
+# always calls it rather than trying to duplicate that check here --
+# a re-run only costs a few seconds once everything's cached.
+echo "running esp-idf install.sh esp32c6 (fast if already installed) ..."
+(cd "$IDF_DIR" && ./install.sh esp32c6)
+
+echo "setup complete -> $IDF_DIR/export.sh (build.sh sources this for you)"