The macos-13 Intel runner no longer gets allocated, so the x86_64-macos release
job queues forever. Ship prebuilt binaries for x86_64-linux and aarch64-macos;
Intel Macs build from source. The install script now says so instead of 404ing
on a missing asset.
install (root) downloads the self-contained joltc release asset for the host
platform, verifies its sha256, and drops the binary in /usr/local/bin (--dir /
--version override). Resolves the latest release via the GitHub API, clears the
macOS quarantine flag, and backs up an existing joltc. Modeled on babashka's
installer. README gets a one-line curl|bash install.