ci: use static matrices with change-based guards

Gitea does not expand a dynamic fromJson matrix (the job name stayed literal
and it queued with no matching runner). Instead, compute cosmic/remote booleans
in a changes job and gate static-matrix build jobs with job-level if.

ci-changes.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+# Given changed paths on stdin, print `cosmic=<bool>` and `remote=<bool>` so the
+# workflow can skip images that didn't change. Shared files or build tooling
+# changes rebuild everything; empty input (schedule/manual) rebuilds everything.
+set -euo pipefail
+
+changed="$(cat || true)"
+
+all=0
+[[ -z "${changed}" ]] && all=1
+grep -qE '^(build\.sh|release\.sh|prune-releases\.sh|prune-registry\.sh|ci-changes\.sh|\.gitea/)' <<< "${changed}" && all=1
+grep -qE '^images/shared/' <<< "${changed}" && all=1
+
+cosmic=false
+remote=false
+if [[ "${all}" == 1 ]]; then
+  cosmic=true
+  remote=true
+fi
+grep -qE '^images/fedora-cosmic/' <<< "${changed}" && cosmic=true
+grep -qE '^images/fedora-remote/' <<< "${changed}" && remote=true
+
+echo "cosmic=${cosmic}"
+echo "remote=${remote}"