Orbit: `/orbit` v2 -- auto-install Orbit from the release feed when it's missing

Follow-up to #124 (v1 shipped in #141).

Today `/orbit` only works if Orbit is already installed -- `findOrbit()` locates an existing binary (`ORBIT_BIN` override, then platform-conventional paths) and `launchOrbit()` spawns it pointed at the current cwd. If Orbit isn't installed, the command has nothing to hand off to.

v2: when `/orbit` is invoked and no Orbit is found, offer to download + install it from the release feed (the same artifacts `release.yml` publishes), then launch. That's the piece that makes `npx @galaxyproject/loom` -> `/orbit` a true universal entry point -- CLI for free everywhere, the richer desktop shell one keystroke away even on a fresh machine.

Open questions:

- Per-platform install UX (dmg / deb / rpm / squirrel) -- prompt and hand off to the OS installer, or silent download-and-run?
- Where to fetch (GitHub release feed) and how to pick the right asset per platform + arch.
- Trust / verification of the downloaded artifact before we run it.