fix(deploy): rename .asset -> .assets so modsharp-deploy bundles gamedata

modsharp-deploy copies .assets/*/ (plural) into the build dir before zipping;
our dir was .asset (singular), so gamedata never shipped with the DLL — a new
DLL referencing a fresh gamedata key hit 'FindAddress: Key ... does not exist'
against the stale on-server file. Renamed dir + fixed README/build.sh/tools refs.
Deploy now logs 'Copying assets' and ships gamedata + DLL together.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>