The Claude Code client lane asserted POSIX, although nothing in it depends
on it. Remove the assertion, tell Windows users to select claude.exe when
the default command cannot be spawned (npm installs put only a claude.cmd
shim on PATH), and record 2.1.296 as verified on native Windows in
docs/testing.md.
Fixes#1923
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>