Review on done: an independent agent checks the exact commits #102

Parent: autonomous-swarm (F5). Inspired by Claude Code peer-programming plugins: when the
implementing agent says done, a hook calls another agent — preferably a different model —
to review exactly what changed.

Why

The queue loop's verify gate already runs a reviewer subagent over git status --porcelain
and git diff HEAD with an LGTM/NEEDS_WORK token (loop/loop.ts ~L1005–1060). Its
limits for the operator's goal:

• the reviewer shares the author's model family unless a persona pins one — same blind spots;
• it reviews the working tree, not a named commit range, so it cannot attest to a head SHA;
• it exists only inside a queue loop — plain opencode and Claude Code sessions get nothing;
• the verdict is not recorded where a merge decision can require it.

What Changes

1. Review trigger on done for any session: loop completion (via loop-done-handoff's
   hook point) and, for Claude Code, a Stop hook running skein review-on-done.
   Triggers only when the session produced commits or a diff since its base and the change
   or branch is not already approved at that SHA.
2. Reviewer selection that differs from the author: prefer a different harness, else a
   different model family, else a different model; routed with the same warm-capacity logic
   as crew work (peer-capacity-subagents, crew-loop 2a). If no independent reviewer is
   available, the review is marked same-model and the policy decides whether that suffices.
3. Exact range, pointers not paste: the request carries base..head SHAs, the worktree
   path, the change slug and acceptance criteria locations. The reviewer reads the range
   itself in a detached worktree at head (no mutation of the author's tree).
4. Verdict as evidence: LGTM/NEEDS_WORK plus findings, written to
   .skein/review.json as {headSHA, base, reviewer{harness,model}, verdict, findings, round}. standing-publish-authority's merge requires LGTM for the current head;
   skein's ReviewVerdict gate evidence reads the same file.
5. Bounded rounds: NEEDS_WORK reopens implement with the findings; at most 3 rounds,
   then the ladder (escalate-before-idle) sends it to the lead. A re-review happens only
   for a new head SHA.
6. Findings rules: the reviewer must cite file and line for each finding and separate
   blocking from advisory; a review with no verdict token is "no verdict", not a pass.

Non-goals

• Not a replacement for CI or the verify gate's test run.
• No auto-fix by the reviewer; it reports.
• No review of a SHA already approved.

Impact

loop/loop.ts (generalise the verify reviewer to the commit range, add selection),
new review/on-done.ts, peer/delegate.ts request type, cli/cmd (review subcommand),
Claude Code hook config fragment, .skein/review.json schema, tests.