ci, install: update workflows, packaging scripts, and installer suite

Nightly/release workflows: matrix tweaks, artifact path fixes.
Package-macos: streamline dmg build, codesigning improvements.
Install scripts: rewrite install.sh and install-nightly.sh with
robust error handling, add uninstall.sh, overhaul update.sh.

ci/verify-version.sh

@@ -5,7 +5,7 @@
 # branch. Runs in the repo root after checkout.
 #
 # Usage:
-#   ./scripts/verify-version.sh \
+#   ./ci/verify-version.sh \
 #       --builds-dir /var/www/builds.skaldagent.net
 #
 # Exit codes: