CI: port breadcrumbs onto bread-ecosystem's shared Arch build image

Adds ci/build.sh + ci/bread-ecosystem.rev (pinned to bread-ecosystem
147cfbb) following the pattern proven in breadpad, so releases build
inside the shared pinned container instead of installing toolchain
deps directly on the bare runner. Routes dev/rc/release build+test
steps through it and adds a check.yml for fast clippy/test feedback
on feature/fix branches. No ci/deps.txt — breadcrumbs' dependencies
(clap/serde/toml/serde_json) are pure Rust with no system library
needs; nmcli/tailscale/sudo/xdg-open are shelled out to, not linked.

ci/build.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+# Delegates to bread-ecosystem's shared CI build image/script, pinned to
+# the commit in ci/bread-ecosystem.rev — not `main`. bread-ecosystem's CI
+# files now affect every product's release pipeline, so bumping the pin
+# is a deliberate act instead of silent drift (see the bread-theme test
+# that broke here for exactly that reason, before it was pinned by rev).
+#
+# Usage: ci/build.sh cargo build --release --locked
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+REV="$(cat "${ROOT}/ci/bread-ecosystem.rev")"
+
+CACHE_DIR="/tmp/bread-ecosystem-ci-${REV}"
+if [ ! -d "$CACHE_DIR" ]; then
+    rm -rf /tmp/bread-ecosystem-ci-*
+    git clone https://git.breadway.dev/Breadway/bread-ecosystem.git "$CACHE_DIR"
+    git -C "$CACHE_DIR" checkout --quiet "$REV"
+fi
+
+bash "${CACHE_DIR}/ci/build.sh" breadcrumbs "$ROOT" "$@"