Release 2.0.0-alpha.2

Rename the CLI to allthings, keeping atw as an alias for now

The package and binary are allthings (cli/), at 2.0.0-alpha.2. The old
name still runs, from the package's atw bin or the installer's atw
symlink, and prints a note on stderr. The site it reads is one value,
siteOrigin in cli/src/config.ts, which stays on allthingsweb.dev until
allthings.dev serves the new site. ALLTHINGS_MCP_URL replaces
ATW_MCP_URL, which is still read. Releases ship allthings-
binaries, and a new installer puts allthings and atw in ~/.allthings/bin,
falling back to the old asset names for earlier releases.