Upstream of brain commits 08a00149 + 971586ef + the arm-6 sandbox fix
(brain 5th commit of 2026-08-28 series):
- R1: -b/--body is the canonical comment flag (matches issue-create,
issue-edit, pr-create, pr-edit); -c/--comment stays a compatible alias.
- R4: usage errors print to stderr and exit 2, distinct from provider,
credential, and verification failures (exit 1). Value-less flags fail
loudly (previously -c with no value died silently at rc=1 via set -e on
the failed shift 2). gh failures on the GitHub path normalize to exit 1
(gh's own usage errors exit 2 and would collide with the reserved code).
- Tests: new usage-contract suite (help rc, unknown/missing/value-less rc=2
on stderr, alias parse acceptance under a sandboxed runner, GitHub-path
exit normalization with a stubbed gh, zero provider contact on parser
failure); readback suite gains case 11 (full verified write via -b) and
neutralizes seat-exported MOSAIC_GIT_IDENTITY / MOSAIC_BRAIN_HOME that
escape the sandboxed HOME (documented HTTP 401 / fail-loud shapes).
Driver: a fleet seat full-stopped on an issue-comment usage error because
usage failures were indistinguishable from provider failures and the stop
gate treated every wrapper failure as blocking.
