Environment-aware dependency probe (linux|darwin|win32|wsl) with a static
registry, an injectable ProbeHost seam for testing, grouped table + `--json`
output, and a non-zero exit when a required dependency is missing/outdated.
Node and tmux are the only hard-required tools; the agent CLIs and document
converters (LibreOffice / MS Office via WSL interop) are optional. CI-safe
unit tests (no tmux, injected host).
