fix(cli): probe Windows PATHEXT variants in find_in_path (#221)

Agent CLIs on Windows are installed as .cmd/.ps1/.exe shims (e.g. opencode
via mise/npm), but find_in_path only probed the bare name, so they weren't
detected. Try .exe/.cmd/.bat/.ps1 per PATH entry on Windows.

Relates to #221 (opencode not detected). The install-time taskkill 'eq'
error from the same report is addressed separately — cbm_kill_other_instances
now uses _spawnvp with an argv array rather than a shell string.