test_command_exists_known fails on Windows (asserts command_exists("ls"))

[dmwyatt]

platform::tests::test_command_exists_known asserts command_exists("ls"), but on Windows command_exists shells out to where.exe, and ls is not a real executable there (it is a PowerShell alias and a Git Bash builtin/coreutils binary that is not necessarily on PATH). The test comment even says "which itself should always exist on unix", so the assertion encodes a unix assumption.

Observed while working #52: cargo test on Windows 11 gives

test platform::tests::test_command_exists_known ... FAILED
assertion failed: command_exists("ls")

CI does not catch this because it runs on Linux.

Fix options: pick a command that exists on both platforms per cfg (e.g. where.exe itself on Windows, ls elsewhere), or gate the current assertion to #[cfg(unix)] with a Windows counterpart.

[dmwyatt]

Extra motivation for fixing this: because the failure is in the grans bin test target, cargo test on Windows aborts before any of the integration test binaries in tests/ run. While working #52 that masked an integration test failure locally (meetings_search_notes_no_match) that only surfaced in CI. Until this is fixed, run cargo test --test <name> explicitly on Windows to exercise the integration suites.