fix(cli): aura graph rejects a bad blueprint arg, not a silent sample (#28)

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

docs/authoring-guide.md

@@ -86,7 +86,10 @@ $ aura graph build < smacross.json
  "input_roles":[{"name":"price",...,"source":"F64"}],"output":[...]}}
 ```
 
-Introspection is build-free wherever possible:
+The built blueprint renders visually, too: `aura graph blueprint.json` emits an
+interactive HTML DAG so a mis-wire is visible before any run (`aura graph` with
+no file renders the built-in sample; a named-but-unreadable file is a usage
+error). Introspection is build-free wherever possible:
 
 ```
 $ aura graph introspect --vocabulary        # one node type per line