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/lib.sh

@@ -775,14 +775,15 @@ except Exception:
     pass
 " 2>/dev/null || echo "")
 
-      if [ -n "$idle_pane" ]; then
+      # An idle pane is reused only when no environment has to be set: the
+      # pane's shell is already running, and `agent start ... -- <args>` hands
+      # every token after -- to the agent executable (--kind), so an
+      # `env K=V` prefix would reach the agent as arguments, not as its
+      # environment (Issue #21 R1). With env flags, fall through to
+      # pane split / workspace create, which take --env.
+      if [ -n "$idle_pane" ] && [ -z "$env_flags" ]; then
         existing_ws="$cand_ws"
         target_pane="$idle_pane"
-        if [ -n "$env_flags" ] && [ -n "$final_cmd" ]; then
-          idle_env="${env_flags//--env /}"
-          idle_env="${idle_env:+$idle_env }HERDR_WORKSPACE_ID=$existing_ws"
-          final_cmd="env $idle_env $final_cmd"
-        fi
         break
       fi