Add CLI-only install script (scripts/install-cli.sh)

[jarugupj]

Summary

Adds scripts/install-cli.sh, a CLI-only installer that installs only the hypeman CLI from kernel/hypeman-cli releases — without attempting the server install.

This is the script side of fixing the get.hypeman.sh/cli path: today /cli redirects to the full install.sh, which installs the server first and aborts on macOS (no darwin_arm64 server release artifact exists), so the CLI never installs. A standalone CLI installer lets /cli install the CLI on any supported machine regardless of server availability.

The redirect flip (pointing get.hypeman.sh/cli at this script) is a separate change in the kernel/hypeman-www repo and should land only after this is merged to main.

What it does

• Detects OS/arch and maps to the CLI release naming (darwin → macos, .zip; otherwise <os>, .tar.gz).
• Resolves the latest kernel/hypeman-cli release that has a matching artifact (reusing the same find_release_with_artifact helper used by install.sh).
• Downloads, extracts, and installs the hypeman binary to /usr/local/bin (falling back to ~/.local/bin when not writable), and warns if the target dir isn't on PATH.
• Requires none of Docker, codesign, systemd, or KVM — those are server-only.

It deliberately does not generate ~/.config/hypeman/cli.yaml (that needs the server's token tooling) and does not carry the server installer's "Intel Macs not supported" guard (the CLI runs fine on Intel and a macos_amd64 build is published).

Test plan

• Ran on Linux x86_64 against the live kernel/hypeman-cli release: installs hypeman and hypeman --version / --help work; the not-on-PATH warning fires when installing outside PATH.
• Verified the macOS arm64 artifact the script resolves (hypeman_<ver>_macos_arm64.zip) exists on the current release.
• Not run on macOS directly (no Apple Silicon in the build env) — the darwin path is exercised by the OS/arch + naming logic above but has not been executed end-to-end on a Mac.