The #28 cycle-close audit found aura graph <file> silently rendered the embedded
sample at exit 0 when the positional named an unreadable blueprint (typo,
nonexistent path, wrong extension): is_blueprint_file returns None both for "no
positional" and for a bad arg, and dispatch_graph's None arm conflated them —
re-introducing the exact #28 "I asked for my graph, got the sample" surprise in
the error path, and diverging from every sibling verb (a bad blueprint arg is a
usage fault, exit 2, per C14). Split the None arm: no positional -> the sample
(unchanged); a positional that is not a readable .json -> a usage error naming
the arg, exit 2, mirroring dispatch_run.
Also close the paired doc drift the audit flagged: the README blueprint-verb
table and the authoring guide now document the aura graph <file> render surface,
absent while graph was the last sample-bound verb.
refs #28
Brummel
962b249814