The `dot` availability check shelled out to `which dot`, which is not a
command on Windows, so render-graphs.js reported graphviz as missing on
Windows even when it was installed. Replace it with a direct `dot -V`
probe via execFileSync.
Also switch the SVG render call from execSync to execFileSync('dot',
['-Tsvg']). Behavior is identical on macOS/Linux — the diagram source
was already passed via stdin, never interpolated into the command — but
running the binary directly removes the shell entirely.
