v0.12.1 — scannable, grouped --help

--help is now scannable and grouped

The help text dumped ~150 lines of dense prose plus a flat alphabetical list of ~45 flags, and printed the entire manual on any command-line typo. This release reworks it (#16):

• Command-line errors now show a short synopsis and point to vexscan -h, instead of the whole manual.
• -h groups flags into labelled sections (Targets, What to check, Reachability, Advisory sources, VEX, Triage, LLM, Output, CI gate, and more) with aligned, wrapped descriptions.
• The top prose is trimmed to a description, synopsis, a handful of curated examples, exit codes, and a README pointer. No information is lost; the deep detail lives in the README.
• Single-letter -V renders with one dash, and --version no longer shows a misleading value placeholder.

No behavior changes: only the help output and flag descriptions.