fix(m171): group help reaches stdout for every invocation, not just a bare one

REVIEW caught the gap: resolving the bare-group case from the argv shape
meant `agentsfleet workspace` printed help on stdout while `agentsfleet
workspace --json` still printed it on stderr — the same inconsistency
this milestone exists to remove, reintroduced one flag away.

The root cause is commander answering an action-less group with
`help({ error: true })`, which writes through writeErr and exits 1.
Overriding `help` on those nodes to force `error: false` fixes it at the
source, so every invocation shape lands on stdout at exit 0 and the
addHelpText tail comes with it. `--help` is untouched — it already calls
help() with no error context.

Adds the regression row the miss earned: every group node asserts its
help reaches stdout with a global flag present, not only bare.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>