Whetuu: zero-config cross-shell prompt in Zig

Whetuu is a zero-config cross-shell prompt written in Zig. It installs to ~/.whetuu/bin, adds two lines to the active shell config, and uses Nerd Font glyphs for git status and toolchain version. The history picker filters by directory, runs subprocesses in parallel, and drops slow probes after 250 ms to keep the shell responsive.