When an SDD implementer commits to the wrong branch (#2050), the
BASE..HEAD range handed to review-package is either empty or not rooted
at BASE. Both cases previously produced a review package silently —
an empty one lets the reviewer approve "clean" work that isn't there.
Add two mechanical guards after BASE/HEAD validation, exiting 3 (vs 2
for usage errors) so callers can distinguish range problems:
- git merge-base --is-ancestor BASE HEAD, else "HEAD is not a
descendant of BASE"
- git rev-list --count BASE..HEAD > 0, else "empty commit range"
Guard shape credits the analysis in closed PR #2082 by @stantheman0128.
Fixes#2050
sdd-workspace now requires the plan file and resolves
.superpowers/sdd/<plan-basename>/; task-brief and review-package write
into their plan's directory (review-package gains PLAN_FILE as its first
argument). Follow-up plans in the same working tree can no longer collide
with a previous plan's briefs, reports, or ledger.
Jesse Vincent