Give classgraph-plugin-flag.sh a real --help

Give classgraph-plugin-flag.sh a real --help

The previous -h/--help ran a sed that quit at the first lone '#' (line 2),
so it printed nothing. Replace it with a usage() that documents every option
(--dir, --cabal, --package, -h/--help), the CLASSGRAPH_SO env var, the
stdout/stderr split, .so discovery + re-run caveat, and examples. Unknown
args now report to stderr with a '--help' hint and exit 2.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>