fix(issue-21): review R1-R3 - no env as agent arguments on a reused pane, one jobs dir for a relative MAM_JOBS_DIR, stop keeps the audit-log root

Review 9a1adb46 (planner-reviewer-codex-01):

- R1 [P1] lib.sh new-session: a forwarded binding reached the agent as
  arguments. With env flags the idle-pane branch prefixed `env K=V ...` to
  the command that `herdr agent start ... --kind <agent> -- <args>` passes to
  the agent executable. An idle pane is now reused only when no environment
  has to be set; otherwise the pane is created by pane split / workspace
  create, which take --env. The dead `env` prefix code is removed (its
  send-text fallback would have typed the same line).
- R2 [P2] runner: a relative MAM_JOBS_DIR is resolved once against the
  control root, the way the wrapper (cwd=workspace_root) reads it, and that
  absolute dir is used for delegation, wait, report lookup and cleanup.
- R3 [P2] stop: the row's control_plane.delegate_job_logs_dir is restored
  (exported unless the caller set DELEGATE_JOB_LOGS_DIR), so the
  cancellation history lands in the row's audit-log root.

Tests: T3-1 and T3-3 now assert that the binding arrives as --env on the
pane-creation calls and that no env/K=V token follows `--` in agent start
(fresh and reused pane); T3-4c (audit root on stop) and T2-15 (relative
MAM_JOBS_DIR) are new. The design doc's compatibility table and a create
comment no longer describe the removed idle-pane env path.

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

.agents/skills/multi-agent-mux-stop/scripts/stop_session.sh

@@ -133,7 +133,8 @@ for s in d.get('herdr_sessions', []):
         cwd = (s.get('pane') or {}).get('cwd', '')
         jid = s.get('delegate_job_id', '') or ''
         cp = s.get('control_plane') or {}
-        print(json.dumps({'cwd': cwd, 'job_id': jid, 'mam_jobs_dir': cp.get('mam_jobs_dir', '') or ''}))
+        print(json.dumps({'cwd': cwd, 'job_id': jid, 'mam_jobs_dir': cp.get('mam_jobs_dir', '') or '',
+                          'delegate_job_logs_dir': cp.get('delegate_job_logs_dir', '') or ''}))
         sys.exit(0)
 sys.exit(7)
 ") || {
@@ -150,6 +151,12 @@ ROW_JOBS_DIR=$(printf '%s' "$MAPPED_DATA" | python3 -c 'import sys,json; print(j
 if [ -z "${MAM_JOBS_DIR:-}" ] && [ -n "$ROW_JOBS_DIR" ]; then
   export MAM_JOBS_DIR="$ROW_JOBS_DIR"
 fi
+# Same for the audit log (R3): the cancellation history belongs next to the
+# registry the row names, not under $WORKSPACE_ROOT/.mam/delegate_job_logs.
+ROW_LOGS_DIR=$(printf '%s' "$MAPPED_DATA" | python3 -c 'import sys,json; print(json.load(sys.stdin).get("delegate_job_logs_dir",""))')
+if [ -z "${DELEGATE_JOB_LOGS_DIR:-}" ] && [ -n "$ROW_LOGS_DIR" ]; then
+  export DELEGATE_JOB_LOGS_DIR="$ROW_LOGS_DIR"
+fi
 DELEGATE_JOB_TOKEN=""
 DELEGATE_JOB_ATTEMPT=""
 if [ -n "$DELEGATE_JOB_ID" ]; then