feat(issue-21): item 5 review identity - per-round snapshot and snapshot diff, detached review worktree (--review-isolation), stale-round re-review, identity records with evidence, pre-promotion re-check; remote reviewers refused or diff-only (--remote-review)

Active with --worktree/--base only; without them prompts, flow and promotion
are unchanged (a loop_id is recorded in the checkpoint either way).

- Every review round is pinned (B3): the source's HEAD and its working state
  as a tree built through a temporary index (the real index/status/refs are
  untouched); a dirty state becomes a ref-less snapshot commit on HEAD
  (committed as mam-loop, not as the user). An untracked nested repository
  cannot be pinned and is refused. identity = (loop_id, base, head, tree).
- Review isolation (B4): a detached review worktree of the snapshot under
  TMPDIR (default), or `--review-isolation env` (the source root with env and
  temp-dir instructions). Failing to open the tree refuses the round - the
  isolation is never lowered silently. Trees are removed after the round and
  on any exit (run() wraps the body; release_guard too).
- The reviewers' diff comes from the snapshot (B13):
  diff_collect.sh takes an optional head commit for a commit-to-commit diff;
  its two-argument form is byte-identical.
- Stale rounds (B7/B8): after the round the source is measured again; if it
  changed, the round's approvals are discarded and the next iteration
  re-reviews without a correction job (last iteration: rc 1). Only the last
  non-stale round's PASSes are promoted.
- Records (B9/B15): review_rounds in the checkpoint with per-reviewer
  transport/evidence, and report-<jid>.identity.json next to each promoted
  report: evidence review-tree | source-tree | diff-only.
- Pre-promotion re-check (B10): a source changed after approval is not
  promoted (rc 1).
- Remote reviewers (B14, Creator challenge 91c81095): they cannot reach this
  host's review tree. Checked before anything is delegated: refused by
  default; `--remote-review diff-only` admits them with the snapshot SHAs and
  diff as their only evidence (no local path in their prompt), and refuses a
  round whose diff is truncated or unavailable.
- run_loop.sh accepts both options (usage identical to the runner's); the
  legacy shell loop refuses them before the lock.

Tests tests/test_issue21_loop_b.py L1-L16 (22).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017R3Eg7MEpSWnm1oB9ZPP6G

.agents/skills/multi-agent-mux-loop/scripts/diff_collect.sh

@@ -55,6 +55,11 @@ _mam_untracked_diff() {
 mam_collect_changes_diff() {
   local repo_root="${1:-}"
   local base_commit="${2:-}"
+  # Optional third argument (Issue #21 item 5): a commit - the review
+  # snapshot - to diff against base, commit to commit. The snapshot already
+  # holds the untracked files, so no untracked scan; nothing falls back to
+  # the live tree.
+  local head_commit="${3:-}"
 
   if [ -z "$repo_root" ]; then
     printf '%s\nreason: repo_root parameter is required\n' "$MAM_DIFF_UNAVAILABLE_MARKER"
@@ -70,6 +75,12 @@ mam_collect_changes_diff() {
   raw_diff=$(
     cd -P "$repo_root" 2>/dev/null || exit 3
     git rev-parse --git-dir >/dev/null 2>&1 || exit 4
+    if [ -n "$head_commit" ]; then
+      git cat-file -e "${base_commit}^{commit}" 2>/dev/null || exit 6
+      git cat-file -e "${head_commit}^{commit}" 2>/dev/null || exit 6
+      git diff "$base_commit" "$head_commit" || exit 5
+      exit 0
+    fi
     if [ -n "$base_commit" ] && git cat-file -e "${base_commit}^{commit}" 2>/dev/null; then
       git diff "$base_commit" || exit 5
     else
@@ -86,6 +97,8 @@ mam_collect_changes_diff() {
       reason="not a git repository: $repo_root"
     elif [ "$rc" -eq 5 ]; then
       reason="git diff command failed"
+    elif [ "$rc" -eq 6 ]; then
+      reason="base '$base_commit' or head '$head_commit' is not a commit"
     fi
     printf '%s\nreason: %s\n' "$MAM_DIFF_UNAVAILABLE_MARKER" "$reason"
     return 2
@@ -104,6 +117,10 @@ mam_collect_changes_diff() {
     local stat_summary=""
     stat_summary=$(
       cd -P "$repo_root" 2>/dev/null || exit 0
+      if [ -n "$head_commit" ]; then
+        git diff "$base_commit" "$head_commit" --stat 2>/dev/null || true
+        exit 0
+      fi
       if [ -n "$base_commit" ] && git cat-file -e "${base_commit}^{commit}" 2>/dev/null; then
         git diff "$base_commit" --stat 2>/dev/null || true
       else
@@ -117,7 +134,11 @@ mam_collect_changes_diff() {
     printf 'The change set is %s bytes / %s lines, over the review limit (%s bytes / %s lines).\n' \
       "$byte_cnt" "$line_cnt" "$MAM_DIFF_MAX_BYTES" "$MAM_DIFF_MAX_LINES"
     printf 'Only the file-level summary is shown below. You have NOT been shown the\n'
-    printf 'full change set -- inspect the working tree directly before voting.\n\n'
+    if [ -n "$head_commit" ]; then
+      printf 'full change set -- inspect the review snapshot (%s) directly before voting.\n\n' "$head_commit"
+    else
+      printf 'full change set -- inspect the working tree directly before voting.\n\n'
+    fi
     printf '%s\n\n' "$stat_summary"
     printf '%s\n' "$MAM_DIFF_TRUNCATED_MARKER"
     return 0